Sitting trot.....well, with ALL the western gait, you CAN NOT grip with your thigh and knee. You must relax your thigh down through your knee, and the "wrap" the lower leg. If your horse has a rough jog, then you CAN work on getting a better jog through training.
Here's something that really helped me when I was trying to sit the trot in western: IF you can't put your stirrups up (I have a horse that is neruotic about different feelings) and practice that way focus on relaxing. For lack of a better word just go limp and allow your body to move of its own accord. When you've relaxed start focusing on your seat. Imagine that you've got concrete bags tied to your butt and it can't move anywhere but forward and back. Don't tense up, just push your center of gravity into your hips/lower back. This has helped me sit a lot of rough horses and also helped me sit the trot when riding english. Another good thing to do (depending on if your horse is tolerant and your willing) is to rider bareback and trot. It puts a whole new perspective when you find your horses stride and can feel it 100%.