If you are bouncing a lot you are probably sitting on your crotch and thighs, like an english rider. What you need to do is sit more on your butt cheeks not just your butt.
Think of your belly and core stretching and contracting, make sure your lower back is really loose too. Kinda make your self feel like jelly but at the same time your covered in rubber bands.
Just a question - how do us english riders (dressage in particular) manage to sit enormous, energetic true extended troton a huge moving warmblood if we sit on our crotch and thighs all the time, making us bounce?
I also suspect the OP is riding english given this is on the dressage section?
I have been drilled from day one, open the thigh, drop the knee, allow the pelvis and engage the core. I want to be able to feel my seat bones on the saddle, as well as lightly on my pubic bone.
Sit trot is always easier on a horse with a soft, swinging back. However, this is difficult to keep if you sit and bounce all over their back.
As an above poster said, start with keeping the trot very slow, get into the feel of sitting into the horse, allow your pelvis to swing in an upwards motion. As soon as you feel that you are starting to bounce, either walk or start rising. Once you're bouncing its near impossible to get your rhythm back, and we never, never NEVER punish a horse in the back. Look after his back, and he'll look after yours :)