Desire and a healthy dose of masochism.
Frankly, there is no foolproof way to prepare for this position. If you want anyone to take you seriously, you'll generally need to have a four-year degree in something. Experience as an assistant somewhere will definitely help, but it better have given you a firm organizational base—you need to be good at finding and keeping track of information, and willing to roll over and do whatever someone tells you to.
You may be working a lot on a computer, so computer skills—or even a computer degree—could be very handy. You should be a social creature, as you'll have to communicate with a diverse (and often tough to deal with) array of individuals.
Flexibility is a must. You'll be moving constantly, and all of your own concerns, relationships, interests, and hobbies are secondary. You'll have days off, but expect to get emails and phone calls even while you're enjoying Christmas with your own family (that never gets to see you anymore).
It's also a good idea to want to be involved with the entertainment industry. Then, after you've gotten the fairy tale knocked out of you by the unreasonable demands, you can move on and do something more substantive and rewarding with your life.