A vid. would be fantastic. I could really judge better with one.
But if you don't have one - that's ok! The sitting trot is difficult to master - especially on a horse that has large strides. My new horse has yet to come to my barn, but when I test rode her I groaned a bit. It's going to be VERY difficult sitting her trot.
Alright, to put into words... You want your general shock absorption to be in your core area - as in your lower back and through your abdomen. A good sitting trot requires very strong core muscles. Perhaps if it would help you could do work outs on the ground to help strengthen these.
When I first began the sitting trot, I did actually tighten with my legs to hold on. I had to truly learn what my horse's sitting trot felt like before knowing how to sit it. My posting trot had been mastered by knowing my horse's movement and allowing his movement and stride to bounce me forward out of the saddle so that posting was effortless. This is when I first learned of relaxation and moving with the horse's body. It may help to know this first. Anyway, being able to lunge while working on your position is fantastic, but I know every doesn't have that luxury - I didn't! It may help to put side reins on your most trustworthy horse so you can focus as much as possible on your seat rather then controlling your horse. Once I had this done comfortably, I asked for a slow, calm and steady trot. It was terrible to watch in the beginning, but I really got a feel for the trot in the saddle. The biggest thing is relaxation. A tense rider will NEVER have a good sitting trot. Once I begin to relax, I ensured that my spine and body alignment was correct. Balance is the second biggest key. To have a balanced seat, your upper body must be directly over your leg. To be tilted or to have your legs too far forward or backwards will throw you off. I can't stress relaxation and balance enough. Proper stirrup length is crucial as well, it may throw off your entire body. Once I found my balance, I began to relax. I began learning how to relax, and as this happened, my legs began to grip less and less. I had learned the true movement of the horse from sloppy sessions before - but regardless, I learned. The pieces fall when one thing is truly achieved.