Okay.. I had a dreadful time with my sitting trot because I get very tight in my hips. That's one of the main problems is the rising trot you can use that "power" to get off of the back, but the sitting trot you want to be glued to the back without slamming. You have to expel that energy out of the bottoms of your feet.
What I tried to do was open up my legs more, and RELAX my hips. Sitting the transition of walk to trot, even the rising trot, really helped me to first try it. Starting at a very slow trot, like a western jog (try to manage the size of trot by the speed of your posting and the amount of leg and seat when you ask. Half halts also help, but don't be rough, gentle aids and your horse will start to get it)
ANYWAY :P you want to sit on the pockets of your jeans (if you were actually wearing them) not on your crotch, no tipping, no 2 point, back. Watch that you don't tense your shoulders or brace your arms. I did this and even after relaxing them, my wrists got tense and as soon as I released, we had 3 perfect strides.
It's about relaxing, and not panicking if you start to bounce.. just rise and get him slow again.
Start small.. don't try to go a full lap around the arena. Right now I'm not engaging my core because I'm still learning how to isolate it properly without bracing any of my other muscles.. but if you can relax those hips, breathe down into your seat, release those knees (it takes a lot of practice), and get rid of the energy out of your feet, you will have a very nice start to the sitting trot. And over time your body will remember how it feels and how it got there, and it'll become cake.
Good luck :)