So you want to save money and be crammed in like a sardine. There are several ways to get cheap airline travel in North America: choose routes where the fares are cheapest; choose times when the routes are cheapest; shopping around; and flying on budget airlines.
How to book, when to fly
Booking in advance often gets you a lower fare. The cheapest fares are likely to be unavailable in the last three or four weeks before a flight.
You can sometimes get cheaper airline tickets by shopping online than by using a travel agent. Some airlines even allow you to pick your seat when booking, and many offer web-only discounts. However, travel agents have access to wholesaler prices, which can sometimes be much cheaper than the published prices of the airlines.
Cheaper fares can often be had for trips that depart between Monday noon and Thursday noon, or on Saturdays. However, spending at least Saturday night at your destination may also help you find a lower fare. This policy, begun by airlines trying to profit from business travelers, is becoming less common. Monday, Friday, and summer Saturday flights tend to be crowded. Check a variety of dates if your travel plans are flexible.
Some people with very flexible schedules (and a lot of patience) save money by consistently choosing crowded flights, which the airlines often overbook. When the gate agent asks for volunteers for a later flight, these travelers pick up travel vouchers for a subsequent flight in exchange for being "bumped" from the flight.
Certain periods around holidays are more expensive and crowded than normal for all forms of commercial travel and accommodation. In the U.S., this includes the Wednesday before Thanksgiving, the four days before Christmas, and the first working days after New Year's. The holidays themselves and "shadow" days (like December 26) are often very light. Also watch out for Memorial Day weekend, college Spring Break season in March and April, and seasonal destinations (beaches in the local summer, ski areas when there's snow, New England and Canada when there are fall leaves to see, college towns when there are graduations happening, certain major sporting events, Las Vegas on weekends and Valentine's Day).
High-traffic routes between major cities are often considerably cheaper than flights to smaller cities. Consider taking public transit or renting a car for the remainder of your journey. High-traffic routes also allow you more choice of departure time and flexibility in case of missed or canceled flights. (For example, to get to Yellowstone, you might wish to fly to Salt Lake City and drive to Yellowstone from there, instead of flying into Jackson Hole.)
Check the flights for all the airports near your destination city. In New York, check fares at LaGuardia Airport, Stewart Airport in Newburgh, Westchester County Airport in White Plains, and Newark-Liberty Airport in New Jersey as well as JFK (the airport code NYC will check fares at all five airports on sites like Travelocity - all airports have easy connections to the city). Long Island MacArthur Airport (LIMA) in Islip, New York also offers flights through its two commercial carriers, Southwest Airlines and U.S Airways and is accessible via the Long Island Railroad. In Chicago, consider flights into Midway Airport and O'Hare, and in San Francisco, take a look at flights to Oakland and San Jose as well. Discount carriers often fly to these alternate airports, which may actually be closer to your final destination.
Buying a round-trip ticket is usually cheaper than buying two one-way tickets, which usually involves using the same two airports for both leaving and returning. But sometimes this is not true. Use a sophisticated cost-comparison site like Orbitz to check.
Sometimes buying a round-trip ticket is cheaper than buying a one-way ticket, even if you are only going in one direction. But some airlines have instituted a "missed flight" fee to prevent people from doing this. Be sure to check before attempting this.
If you're flying from one coast to the other, consider a "red-eye" flight that leaves around 11:00PM and arrives in the morning on the other coast. These flights are often cheaper, and you spend one less night in a hotel.
Avoid booking the last flight of the day in case you miss your scheduled flight or it is delayed or canceled due to weather or technical problems (neither are uncommon). Airlines do not provide lodging for delays caused by weather or other factors outside their control.
In general, the competition is steeper, and thus prices lower, in the American flight market than the Canadian one. Travellers heading to a Canadian city close to the US border should consider flying into a nearby American airport (the best examples being Seattle for Vancouver and Buffalo for Toronto). In some perverse cases, it may even be cheaper to get between Canadian cities by buying a connecting flight to a US city and doubling back (e.g., flying Toronto-Vancouver-Seattle, then taking a bus back to Vancouver).
Airfares into Essential Air Service airports - usually more rural parts of the country or coastal islands - are government subsidized and usually very affordable whether you buy six hours or six months in advance.
Price comparison websites
The best way to find a cheap fare quickly is to use a price comparison site. These allow you to check multiple airlines, possibly at multiple airports on multiple days. However it often pays to compare the offer with the price on the airline's webpage. Airlines are more likely to offer an electronic ticket and not charge handling fees. You may also be able to search for a hotel or hostel, rental car, cruise, rail pass, vacation package, or other related items at the same site.
Most of these search engines have the option to send you an email when the fares for specific destinations change by a specific amount or go below a specific value.
Note that most price comparison sites do not list fares for low-cost airlines and some major airlines (such as American, U.S. Airways, Southwest, SpiritAir, etc). On the other hand, there are websites targeting low-cost airlines in particular in addition to traditional carriers.
Some airlines such as Continental and American have instituted a guaranteed lowest fare feature on their websites. This means that you are guaranteed to get the lowest fare for that flight on their website. This does not mean that you will not find a lower fare on another airline, but it is an improvement over the past because airlines' own websites offer frequent flyer booking bonuses for using their sites to book tickets. For example, if a multi-airline search on Travelocity showed that Continental had the cheapest flight, go to Continental´s site and book your ticket there to get the bonus. Even if Continental is no longer offering it, you will be able to get the frequent flyer booking bonus.
Check discount sites individually for deals. But discount airlines sometimes have special procedures or restrictions, and may be less willing to allow you to change to a different flight or to give you a refund. Be sure you understand all special terms before booking. Discount airlines are constantly expanding the list of airports they service; check their official sites for details.
Largest Low Cost Carriers
Southwest Airlines  - By far the largest low-cost airline, and in terms of passengers carried, it's nearly as large as any other American airline. No reserved seats. Boarding priority depends on how early you check in online (up to 24 hours before departure) or at the airport. Serves 97 destinations as of May 2012, after merging with AirTran Airways, another low-cost carrier that had hubs in Atlanta, Baltimore, Milwaukee, and Orlando. You may still see planes with AirTran markings, as not all have been converted to Southwest's color scheme, but as of March 2012, all of the company's planes operate under Southwest's certificate. Does not publish its fares on other web sites—all bookings are done through its own web site.
JetBlue  - All flights have DirecTV and wide leather seating for every passenger. Many flights are non-stop. Hubbed in JFK, but with focus cities such as Long Beach(secondary airport,smaller alternative to LAX),Oakland, Boston, Washington Dulles, Orlando, and Fort Lauderdale.
Frontier Airlines  - Low-cost airline based in Denver, with flights from Denver to most major cities throughout the country, particularly in the western United States. Also serves several Mexican resort destinations.
Spirit Airlines  Hubs in Fort Lauderdale, Florida and Detroit, Michigan. Very low fares (sometimes as little as $9 one way) can be found but they charge extra for everything from printing a boarding pass (you must do it online to avoid the fee), to advance seat assignment, and even carry on bags.
Sun Country Airlines  Hub in Minneapolis/St. Paul, MN.
Virgin America  - Based in San Francisco, Virgin flies to Boston, Chicago, Dallas, New York and Washington, DC as well as to Seattle, Las Vegas and several destinations in Southern California, Florida and Mexico.
WestJet Airlines  - Many Canadian destinations and some service to the US and parts of the Caribbean.
Volaris Airlines  - Hubs in Toluca (Mexico City) and Tijuana. Flies throughout Mexico and recently began service to Oakland, Los Angeles and San Jose with plans to expand throughout the United States. A codeshare agreement with Southwest Airlines is planned for late 2010.
Discount airlines with more limited routes (including scheduled charter airlines):
Air Transat  - A Canadian airline serving destinations in Europe, the Caribbean, Latin America, and Florida.
Allegiant Air  - Mainly serving vacation destinations from small cities with limited airline service. Does not offer connecting flight service (focus cities in Las Vegas, Phoenix-Mesa, Bellingham, Orlando, St. Petersburg, and Fort Lauderdale).
Streamline Air  - commuter oriented weekday service between Boston-Hanscom (BED) and Trenton (TTN) near Philadelphia. Frequently the lowest priced option between the two cities, and both airports are small and well linked to Boston and Philadelphia respectively.
Vision Holidays  - Mesa, Arizona, to North Las Vegas, plus Grand Canyon aerial tours.
Many airlines offer student and senior citizen discounts, and have low or free fares for young children.
Most airlines will give you a discounted fare if you must fly on short notice because of a family emergency, such as a death or sudden illness. Call ahead or speak to an agent; this is not possible to arrange online.
Some areas of the United States have special flight and accommodation deals (like Hawaii and Alaska).
These programs are being phased out, and no longer make much sense anyway because the fare structures have changed so much. Full Y fares have come down so much, and most of the time, you are no longer restricted to full Y anyway, even when purchasing an hour before. An article has been written about this in one of the major newspapers.
Military and airline personnel are usually eligible for cheap or free travel on their respective aircraft.
Online booking site offering special military discounts:
MilitaryFares.com  - Domestic, international and intercontinental booking site for active Military Personnel and their spouses only.
bookholiday.org  - Domestic, international and intercontinental booking site for cheap travel.
First & Business Class
Planetamex.com  - American Express Representatives. Specializing in international First Class Airfare and Business Class Airfare at a discounted fare.
"Flying standby" is when you have a ticket for a certain destination, but you don't have a seat reserved on any particular flight. You are allowed to board the next flight going to your destination that has a free seat.
Standby policies vary greatly between airlines.
If you can get them, standby tickets are sometimes cheaper than regular fares if you must book your flight at the last minute.
You may be required to pay an extra fee to fly standby.
If you buy a discount ticket, or a ticket on a discount airline, you are more likely to have to pay a fee or upgrade to a higher fare to change your flight at the last minute (or at all), even if you are just trying to fly standby.
Frequent flyer club members, students, seniors, airline employees, and other special groups are more likely to be able to fly standby inexpensively or for free.
If you hold a confirmed ticket for a later flight on a given day, many airlines will allow a passenger to try to standby for an earlier flight that day for a charge much lower than the change fee (often $50 instead of $150 or higher, and occasionally free.)
Arriving at the airport
Arrive to the airport on time (usually 2 hours ahead for domestic and 3 hours for international) as recommended by your airline and check in right away.
Many airlines have a certain time at which they will no longer check in passengers, because it will take too long for them to get through security or for their luggage to be processed/loaded onto the aircraft, perhaps up to 30 minutes before takeoff. If you are lucky, you will get booked on the next flight for free, but don't count on it. Check with your airline if you might be delayed or if you are habitually late.
Even if you check in for your flight "on time", your airline has no obligation to get you through security and onto your flight. If the airport is crowded, your flight may depart "on time" while you are in the security line. This is uncommon, but it happens. Most likely you will be offered a standby seat on the next flight, but no apology from the airline.
If the flight is overbooked, you may be "bumped." Some airlines intentionally overbook flights, anticipating no-shows. This sometimes results in having to bump passengers to the next flight. When needed, though, airlines typically look for volunteers by offering vouchers towards future flights, sometimes worth $100 or more. To be "bumped" and receive a voucher, though, you still must be present at the gate and talking to the agents. If you're stuck in security or simply late, you'll be treated as a no-show.
If there is space available, some airlines do allow you to take an earlier flight if you happen to arrive early. This might be for free, a small fee, or a large fee. But it can't hurt to ask. Have your airline's reservation number handy, as you can usually make these arrangements on the phone.
This is a usable article. It touches on all the major areas of the topic. An adventurous person could use this article, but please plunge forward and help it grow!