proper sitting trot is the most difficult gait for most horseback riders. A proper sitting trot takes time, practice, a forgiving horse, and, perhaps most importantly, an ability to relax. Here are a few tips to help you perform the proper sitting trot:
Don't force your heels down in proper sitting trot, but make your ankles as loose and relaxed as possible. Loose ankles can absorb a lot of the shock before it ever gets to your body. In proper sitting trot, you don't want a lot of weight in your stirrups, but you DO want to be using them enough that your loose, relaxed ankles can absorb some of the impact.
Use your abdominal muscles to absorb more of the impact. Make sure your shoulders are back and your chest is open, try holding a very slight ab crunch when sitting. Concentrate on allowing the outer muscles in your thigh to stretch and wrap around the horse.
One of the classical exercises to teach you to move with your horse at proper sitting trot is to put your horse in a trot, then recline back to a 45 degree angle, maintaining the trot. (careful the first time to see how your horse responds. green horses may respond unpredictably to this.) When you're leaned back you literally feel like your spine is velcroed to your horse and you will feel every movement of the trot and your own body is forced to move with the horse in that position. When you feel like you've got the movement down sit up slowly (with your abs) trying to maintain that movement. When you lose the rhythm, lean back down.
One final thing that's really helpful to improve horse you perform a proper sitting trot. Before you start working on proper sitting trot: do this exercise. It helps settle your seat bones and open your hips- which makes proper sitting trot a lot easier: