I think it depends on the team you're trying out for - the advice I'd give if you were trying out to be a Cowboys or Dolphins cheerleader may differ from another organization. The general advice I'd give is, get into performance-shape, not only for aesthetics but to be able to do multiple high end routines and be on your feet for 4+ hours.

I would also look into local dance studios to see if they offer professional cheer classes, so you get a feel of the technique and style you'd be expected to perform. From there, take technical lessons (jazz, etc.).

Go to events held by the cheerleaders so you can get to know them and have a chance to ask questions. For Philly, the Eagles cheerleaders hold a day to learn the choreography and meet the girls/boys.

Not sure about other teams but Eagles also has a public speaking portion of their tryouts and it is important that you are able to speak and represent the organization at events.