The simple answer is as much experience as you can get in sophisticated, complex aircraft in a challenging operation where you have to make decisions and interact with all sorts of people. There are many ways to define that last sentence and no two will define it the same way. It's your journey, and task, to put your own meaning to it, and that's why they call it the game of life. The only thing I can guarantee you is that no matter how well you plan it out, things have a way of not going along with your plans. That's what keeps it all interesting.
The best advice anybody can give you is to not be "stupid". No dui's, no trouble of any kind, say your prayers, eat your vegetables, try not to hack anybody off (because it's a small world out there) and enjoy the ride. The most important thing to remember: nothing is guaranteed.
Good luck!