I had a very long struggle with this myself and I used several tricks:
- the thing that helped me is mastering it at an extremely slow, collected trot. Some horses associate the sitting trot with a canter transition so make sure you slow him before you sit.
- relax your hip completely! Don't think about them. Only focus on putting your heels down, and stretching tall tall tall
- do it without stirrups! Even if you can only get in a couple strides before you have to transition to a walk.
- I read it in some forum at one point and it works... sing the oompa loompa song in your head
- alternate putting more weight in left heel/right heel, instead of posting up and down.
- post a slow, collect trot, and make your up/down smaller, smaller until your in the saddle
If you can trust your horse in an arena, I find it always helped me learn to sit a trot/canter by closing my eyes, relaxing, stretching tall, and imagining me being one with the horse. I know it sounds super hippie... but to each their own.
And remember: practise! Do at least 2 laps of sitting trot each riding session (even if you have to walk to rebalance yourself in between)
If your hips are tight, try rotating them and loosening them first (unless your horse isn't the type to like that sort of stuff of course :))