Sitting the trot is a bit like trying to pat your head and rub your tummy at the same time - you have to relax your leg muscles so that your grip is not pushing your seat out of the saddle, while using your abdominal and back muscles to absorb the motion and to follow it.
It helps to start on a lunge line with stirrups and leathers removed. Crossing them over the pommel usually leaves an uncomfortable lump under your thigh.
You want to be sitting as deeply as possible, so do some stretching exercises before you mount - particularly a sideways lunge, where you start with feet apart and slowly transfer most of your weight to one foot, bending the knee and stretching with the other leg straight. You should feel your inner thigh muscles protest a bit. After enough repetitions you should also feel as if your pelvis has spread a bit wider.
Once mounted, place your palms on the pommel and push yourself up out of the saddle, at the same spreading your legs in a wide "V". Lower your seat back into the saddle with your legs still stretched and feel the difference in your seat.
Next grasp the pommel (or a strap running through the "D" rings if that's more comfortable) and pull yourself even further down and forward into the saddle. Now you're ready to sit the trot, in accordance with the advice given in other posts here.
At first aim for only a few strides before repositioning yourself. Gradually add more strides, but always stop with the first bounce and regroup. This will be easier on both you and the horse.
Stop as soon as you feel yourself starting to bounce. Once you begin to bump with every stride your horse's back will stiffen in self defense and the bouncing just gets worse.
Taken from:
How to Sit the Trot - Learn the Sitting Trot
Sarah