If you're not familiar with the David Concertino, you should give it a look. It's chosen as an audition piece for countless music schools and competitions, probably because it showcases many aspects of a trombone player's abilities. I realize that it's beyond some player's abilities, but it's a great piece and is a good gauge to use to tell where you are and what you should work on. Just chip away at it - play and perfect the parts you can play and gradually add the parts that you have trouble with as you get better. Then, when it's time to play it for an audition, it'll be ready and you'll know that you're able to play the things you need to have under your belt for whatever it is you're auditioning for.
Some things this piece includes:
High notes
Low notes
Dotted eighth & sixteenth vs. triplets
Dynamite variations
Articulation variations
Lip trill
