the only way to really master the sitting trot is practice practice PRACTICE! lol
whey you do the sitting trot lift up your belly and really sit on your bum and breath... sing if you have to but if you are holding your breath you are probably tensing up causing you to bounce so just try your best to relaxe into the saddle and breath. try and absobe the bounce with your hips and kind of roll them to do that (with teh movement of the horses trot). practice practice is the only way to really fix it though

maybe at the begining of the ride you can drop your stirrups and relax at the walk than do a little sitting trot. sometimes it will relax the horse and back them off before rides