Here's the way I used to help students with it. I would have them watch my dog's back move at the trot, and really pay attention the the way the animal's back muscles move; that when a hind leg comes forward and pushes off, the back muscles on that side rise, and the other side falls.
Sitting the trot well requires that you allow your seat bones to follow that motion, that your left seat bone rises and your right set bone drops as the left hind comes forward. Any squeezing or gripping trying to hold yourself in the saddle produces that familiar bone jarring pounding.
Most common mistake is that by trying to be still, riders stiffen, then bounce
Being lunged or briefly grabbing the pommel to pull yourself into the saddle to feel the motion is a big help.
Correct position, with your seat bones under you, your back flat and relaxed, neither leaning forward or back makes it much easier.