I would do what K-Swiss said. Try to learn and understand their technique. Also, even if you feel that you aren't ready for Phantom, it never hurts to go to an audition camp. Not only will you get one-on-one instruction from the Phantom staff, but you'll also get an "Audition Packet" which is a thick booklet of past and present warm-ups and exercises. So even if you didn't make it this time around, you will gain SO much knowledge and understanding just from going to one camp.
As far as what they look for. Sure experience is great, and you can get that from marching other DCI corps, or by marching an all age corps. But what they really want are well rounded individuals who have the ability to be molded. Don't ever walk into an audition with a chip on your shoulder(and if an instructor asks to you change something with your technique, never ever look at them and say "well this is how I was taught by my old instructor".), because I can bet you anything that they won't ask you to return(even if you're the best one there). Always be positive, eager to learn, own up to your mistakes, and if an instructor asks you to do something out side of your "comfort zone", make sure that you do it right and do it immediately.
Just remember that Drum Corps is an amazing activity. If you're not having fun, then you're not doing it right!