If you want to improve sitting trot, get somebody to lunge your horse with you on it, that way, you can work on your balance, and not have to worry about your horse.
Some great things to remember when sitting to the trot without stirrups is:
Long and Loose legs - tight legs will just make you bounce around even more.
Lean back a bit - it will help keep you balanced and bit comfier, lets face it, bouncy horses are not fun to sit to!
Sit deep in the saddle - for balance.
If you get lunged on your horse, you can also hold onto the horn of the saddle, and pull yourself down and hold yourself that you can sort of get the feel a bit better.