I have an OTTB who has a massive trot.  I've found that if I warm up WTC (rising trot in warmup), his trot after a canter is better in so many ways.  His back loosens, I am loosened, and we feel good.  That's a good time to address the sitting trot; never on a cold- backed horse!  And the hips/ seat advice is worth its weight in gold.  Dread not the sitting trot- once you master it you will grin, I promise.  Best of luck.

I have found one thing that really helps is focusing on "turning on your core." Kinda like the previous poster said about using your abs (although, personally, I think that doing ab exercises the night before to the point of soreness would make it more diffiuclt for me...but, to each his own). Anyways, really think about bearing down with your abs - almost like the feeling you get when you sneeze. If you really focus on using your core, it should help you soften and balance. Also, try to keep your horse at a steady gait, not too quick, in order to reduce the bounce.

Oh, and one more thing - I can't atest to it myself, but I have heard that Thinline saddle pads can make it easier for the rider to sit the trot. Obviously, it's more important to work on your own position, but this might give you some extra help.

I leaned how to sit trots in a hunt seat class when my instructor decided to introduce me to Western riding. That's how I learned. This was in 1981. Since then, I have lost the ability to post a trot. Now as an older rider, I can only do limited trotting, and I can't sustain a trot. It was better for me, since I do tend to brace sometimes to have the horse do a surprise trot (one I didn't ask for). One of the Percheron sport horses I used to ride threw in a surprise trot a few time. I think he wanted to show me that I could sit his bouncy trots in relative safety.

Now with the Appy I ride, I have had him trot a few times. I sit them just fine. I believe that the problem is the anticipation aspect which breeds bracing. It is really hard to force yourself to relax. I had to come up with a gimick. For me this was a thing called the Other Seat. I used to use a neoprene thing on the saddle a few years ago. The "saddle sticky thing" has given me permission to relax. Who know why I was prone to bracing when I anticipated trotting. I don't know, (maybe the accident I had at 19??) but I found a way to cope with the anticipation part.

Thanks everyone. It's amazing how much you don't relax when keep screaming at yourself to relax. Haha. I worked on it this weekend, doing it little by little. When I felt it get out of control, I went back to posting for a while and then attempted it again. It's getting there, little by little.

 Yes, I can vouch for the Thinline pads! They do make a world of difference if you're feeling any discomfort.