Here's what worked for my students -
The first thing that needs to happen is that you need to understand how the horse's back moves at the trot - try watching an unsaddled horse on the lunge line. As a hind leg comes forward and pushes off, the back on that side rises and the opposite side drops. If students have trouble seeing it on the lunge line, I would call my dog into the ring and have them watch the dog's back as it trotted to demonstrate the concept.
If you grip and try to hold yourself still against that motion; you'll just beat yourself and the horse up. (That's what many riders do sitting the trot.
Now locate a drawing or a model of the human skeleton and locate the seatbones or ischial tuberosities. Then find your own seatbones - they're not where you think they are. Do this in privacy as you'll be groping around your crotch and butt trying to find those bony prominences. It's okay - you need to have a clear idea in your head exactly where the are because in the saddle, you'll want to be sitting erect but relaxed, with your seatbones pointed straight down.
In order to sit the trot, your seatbones must follow
the motion of the back that you observed on the lungeline or on the dog. As one side of the horse's back sinks, the seatbone on that side must sink with it and the other side must rise as the back rises.
It does help to have a slow, steady trot and to be grasping the pommel to pull your seat into the saddle if you lose the following motion.
Another problem novice riders have is that they've been working very hard on developing muscle strength in their legs; and gripping with the legs is the enemy of sitting trot. You can actually pull your entire leg away from the horse's side and just feel the trot in your seatbones. Gripping with your leg, and espeicially with your knee and thight squeezes you up off the saddle and makes it hard to feel the motion.