The key is to sink deep into the seat of your saddle, and allow the weight to flow through to your heels. Think dressage when you do this, and how those riders look while sitting the trot - how they almost lean back (but don't), their shoulders rolled back.
It takes a lot of work with the abdominal muscles. In fact, if you can't feel your abs being worked out almost immediately after starting to sit the trot, you're not doing it right! Another important note is to close your hips. When people begin learning to sit the trot, they often do all the wrong things: they round their backs the wrong way and tip forward, causing their knees and hips to open up, making their legs bounce all over the place and causing instability in the saddle.
These are the two things that I think of when I sit the trot:
- sit deep, straight up, and roll your shoulders back (but don't actually lean back) remember, think dressage, and use your abs
- keep your thigh on (if your thighs are not making 100% contact with saddle, you will loose all stability in your seat and lower leg, causing bouncing with both seat and heels).
I find that if I can do these two simple things, everything else falls into place. My legs don't bounce, my hands stay still, and my horse PUSHES through. In fact, I totally understand why dressage riders sit the trot - it's gotta be THE best gait to get a horse working through their hind end.