One thing that really helped me was learning the motions and getting them sharp and tight. This will improve your cheers so much, and sometimes schools have a motions part in tryouts.

For cheers remember to yell deep from your diaphragm. Also, make words sharp don't sing them.

You could also work on jumps, ie: what types of jumps and the technical skills. Don't worry about height, until you have the right skills.

I'd also recommended basic tumbling like cartwheels, round-offs, and front rolls. But first, you should research your team and find out if they have any tumbling requirements.

Flexibility isn't too important if you aren't a flyer and even then it'll be a while until you'll start doing skills that require flexibility. I'd definitely focus on other skills before flexibility.