Some things that always help me:
Don't sit until the horse is round and over its back. No need to torture yourselves.
Ride without stirrups. For a very long time. Let your legs fall, they are spaghetti, pretend they don't exist.
Ride on the lunge line.
Take your legs off the horse completely, swing your legs around, move them independently, learn to sit on your behind, not your thighs or knees or calves.
Do not tilt or perch, sit on your behind.
Move your shoulders, move your neck, be loose, let your body move with the horse, sing songs, do yoga breathing, pretend you are dancing, pretend you are massaging your horse's back, imaging your hips opening up, find the downbeat of your horse's footfalls and sit to that. Experiment with your core muscles - what happens when you tighten? When you expand? How do they move? Depending on your body and your horse, a good sitting trot might feel like getting punched in the stomach for a while because you are holding yourself up with your core, not your legs, and that core is *moving* - you are not just sitting with a straight, immobile back, that isn't going to work at all. It doesn't matter what you look like, just find a way to move that gives you a secure and flexible seat and makes you move with your horse. You can make it pretty later.
Trust your horse!
Last edited by plomme; 05-18-2013 at 02:43 AM.