The most important thing you can do to prepare to sail is SAIL! Sail anything you can afford or talk your way onto, anywhere you can. There's absolutely no substitute for experience when you're offshore.
With the advent of GPS and tricked out cruising boats with electric everything, I've seen way too many people sailing in places they have no business being on boats they can't handle if anything goes wrong. Way to many cruisers long on $$ and short on experience out there. Learn to reef, steer, navigate, work on an engine, splice, sew, understand weather... the list goes on, all things that only experience can get you.
While Ellen MacArthur sailed early on as crew on an open 60, she also owned several smaller boats that she bought herself (the first by saving her school lunch money). At age 18 she sailed round Britain in a 21' boat, the third she had bought herself and fixed up. She had good singlehanded ocean experience by the time she did the mini-transat a year or so later, and even more before she started singlehanding the big sleds.
The point is, get out and sail as much as you can. Buy a good boat that you can afford and sail it. Crew on other peoples boats. Help them work on them too! And have fun! Good luck!