Apart from making your way round the Caribbean or Polynesia by offering a hand onboard yachts which seems to be common and easy, the most common route is the Atlantic crossing from Europe to the Americas (mostly to the Caribbean or Brazil).
Experience: Experience in sailing is not necessary -although a huge plus in getting a quicker lift-, but participating in duties and life on board of course is obligatory!
Some money: These days most yachts will ask you to chip in for your food. Most boats ask either 10 or €15 on food per day. The Atlantic crossing takes between 15 to 25, depending on boat and winds, so you'll need around 150 to €300 for the crossing alone -count on at least a week in the marina till you find your lift, too. It is possible to get an entirely free lift food included, mostly on bigger yachts where you'l be needed to scrub the deck and polish the silver railing. During the ARC regatta you can even expect boats ask as much as €50 per day since inscription fees are pretty high.
Things you'll need: Waterproofs and good shoes are recommended although they are very expensive and if you don’t have them you can wait -the boat you get might provide them and only if not you'll have to acquire them. The Canary Islands are cheaper when compared with mainland Europe and in Gibraltar for example apparantly there are sailor’s “jumble sales” so you might pay less.
Getting started: Be assured that once you hit the marinas everything will be pretty obvious. You will meet other boat hitchhikers and they will share their information with you. Basically you'll be putting up notices offering your help, pacing the docks approaching people cleaning their yachts, trying to make contact with sailors in the bar etc. Try to talk to as many people as possible. After a while everyone will know you and will give you hints as to which boat is looking for someone.
Language: Obviously all foreign languages can help, but for the Atlantic crossing speaking at least some French can make a huge difference, since over 60 % of sailors on this route are French-speakers
There are stories floating around of people who always know someone else who managed to go for free on a freighter, but the only reliable stories really date back to the seventies. Cargoship travelling is commercialised now virtually everywhere. For something like 50 pounds per day you can rent a cabin on them.
Online: There a several online marine crew websites available that specialise in matching crew with boats. findacrew.net is a worldwide network of mariners and is the largest of them all. There is also a community called floatplan.com, where skippers and crew meet.
When to go: Boats go with the trade winds that start to move from East to West across the Atlantic in autumn. So the season is from September to January-February. Top season is October, November. In the end of November each year there is a regatta called ARC (Atlantic Rally for Cruisers) leaving from Las Palmas. There will be more boats than at any other time in the marinas and it can be considered safer than leaving with a boat that is going unassisted. There will be more competition on finding a lift though, too.
Be aware that for the past three to five years the winds have started to go haywire a little bit, acting less predictably with more storms happening. This is most probably because of global climate change. The way most sailors comment on this is "the winds are going through a transitional phase to find a new rhythm".
If you don’t want to pay the ferry in Algeciras it is possible to sail to Africa, although unless you are extraordinairily lucky, you’ll have to go via the Cape Verde islands which are a stopover for many trans-atlantic sailors. They are between 10 and 14 days from Gibraltar. From there you’ll have to catch a new boat to get to Senegal which is three days away. It is an experience in and of itself although maybe not the perfect swap for a one-and-a-half hour long ferry ride that’ll cost you €25 You also miss out on Morocco and the crossing of the Sahara Desert, which are highlights of any visit to Africa.
From Marocco: Essaouira is your best bet, you can also try Agadir. No one crosses over directly from here though, all boats will be going somewhere in the vicinity (Canaries or Senegal for example) with other plans put up for later.
From Senegal: Dakar or Casamance. With a very likely stopover on the Cape Verde islands.
If you want to take a flight to the Cape Verde islands: the biggest marina is in Mindelo on Sao Vicente, the second biggest one is on Sal where the international airport is.
Since sub-Saharan Africa is out of the influence of the trade winds you can hitch from Senegal almost all year round, although the main bulk of boats will be leaving when it is top season in Europe, that is October to December.
There are three sailing clubs in Dakar. “La Voile d’Or” with shallow water which therefore attracts catamarans only, the “CVD” (Club des Voiliers Dakar) where the majority of boats can be found and a third one also in the vicinity of the two ones mentioned. They are all situated not far from another to the South of the “Cap Vert” peninsula and the town centre. If you try to hitch from there it is strongly recommended you speak some French.
The Pacific being about 6 times larger than the Atlantic, this is a crossing that is never done without a stop over on at least one of the Polynesian Islands. The most likely end-point for most is Australia or New Zealand. Boats begin to leave the west coast of Mexico for French Polynesia in March and continue to sail as late as June and July. The cyclone season, or off season, in the South Pacific begins in November. The initial crossing from Mexico to French Polynesia is roughly 2,500 miles and will take 3-4 weeks on an average sized sailboat that doesn't carry much fuel. Times vary depending on the length of time a boat is stuck in the doldrums or intertropical convergence zone (ITCZ).