Travel for fun is a fairly new concept: for millenia, as today, people traveled mostly out of necessity on business.
Sales is the classic high-travel occupation, so much so that "traveling salesman" has become a bit of a cliche. Consulting can also mean high travel, but on a somewhat less intense scale, as individual consulting gigs tend to last weeks or months and can easily turn into long-distance commuting. In general, any very specialized job, where customers are few but can afford to (or have no choice but to) fly in experts will tend to mean high travel.
The airline industry also offers good travel opportunities. Aside from the obvious pilots and cabin staff, maintenance crew and sales & marketing people may also fly extensively, and even desk job employees can often fly for free or very cheaply using space-available seats.
Travel to exotic locales, staying in quality hotels, maybe even flying business class may sound like an all-expenses-paid vacation. But it's not: in the end, business travel often boils down to the stress of working combined with the hassle of travel, only now you'll often be working in an unfamiliar environment without the ability to walk down to your colleague's cubicle and ask for advice. You are only rarely in control of your own schedule. Being on the road constantly can have an adverse impact on:
relationships — you may not be able to see your family or significant other while traveling
health — practicing sports and eating well is harder when traveling, but the risk of picking up bugs your immune system isn't equipped to handle is higher
levels of stress — after a bad day at work, imagine going to the airport only to find that your flight has been cancelled
If offered a high-travel job, think about it carefully. It can be an interesting experience when young and single, but it can quickly become a drag.
Of course, business travel does have its positive aspects.
It's free. Flights, hotels, taxis, departures taxes, whatever, they're the company's headache, not yours. But beware of the penny-pinching if not outright sadistic travel policy, and fill out your expense reports carefully.
All expenses paid. Many companies offer per diems, where you get paid a fixed amount every day based on your destination, and it's up to you how to spend it. The frugal traveller can actually turn this in a nice little bonus. (The other end of the spectrum, however, is the company that won't pay you anything without an itemized receipt.)
Frequent flyer miles. Your company pays for the tickets, but it's who you will rack up the miles. Sitting on an airplane in your free time may lose its appeal after a while, but you can also use the miles for upgrades or flying down your friends and family to visit you.
New challenges, new experiences. Business travel is still travel, and you will encounter new people, new things and new situations that are guaranteed to be a learning experience and change the way you think.
Before you travel
Find a good travel agent. Booking online can be cheap and easy, but a good travel agent can be worth their weight in gold when your Jumbo flight is cancelled and you need to rebook in a hurry.
Learn the tricks of the trade. Your ticket says you need to show up at the airport three hours before departure, but maybe 45 minutes will do in a pinch.
Have a packing routine. Invest in a good carry-on bag and learn to pack enough to survive a week with it. Figure out the optimal way to pack it, because when everything has its place, it's easy and fast to pack. If you often travel on short notice, consider keeping the bag packed and ready to go.
Mileage, mileage, mileage. You probably know you can get miles from flying — but you can also get them from staying at hotels and renting cars, and if you pay by credit card, you can get more miles yet again. Familiarize yourself with the programs at places you visit regularly and work out how to maximize your benefit. Dedicated sites like FlyerTalk are useful for working out the loopholes and finding the latest promotions.
On the road
Learn the language. Even a few words will smooth your way and you can pick the survival-level basics of most languages in a few weeks if you take some time to study.
Work out. Most business-level hotels have a gym and any hotel's front desk will be happy to advise a good jogging route nearby.
Get out of the hotel. It's all too easy to sit in your hotel room, order overpriced room service, and grumble how miserable the dump you're in is. Ask a local (or check Wikitravel!) for a recommendation and go for dinner or a drink elsewhere.
Find a local friend. The Internet is full of friend-finding and online dating services, and many people will gladly take a visitor for a tour of the sights, even if you're only in town for a day or two — just offer to return the favor when they come your way.
Spend the weekend. If your trip starts on a Monday or ends on a Friday, spend the weekend sightseeing. You're already there so the additional effort involved is minimal, and you'll see more than the airport, hotel and office at your destination. If you're spending more than a week away, most companies will be more than happy to pay your hotel and expenses instead of flying you back for the weekend.