We've ALL been there, so don't feel like its just you. ;)
Tai Chi and Yoga really helped my seat for dressage. The important part is to increase the flexibility in your hips and thighs. The tendons/ligaments/muscles that allow you to do the splits, and the ones that control twisting/turning, plus the ones the allow you to bring your thighs under you (quadriceps). When those are tight (and they are for all of us), it causes your legs to slide up when you ride. The more limber and stretchy you can become, the easier it'll become to get your thighs/knees/calves under your hips.
I do quadricep stretches, hip flexor stretches, lower back stretches: (Useful Stretches for Riders
I was trying to find a good Tai Chi video that would demonstrate (somewhat) what I'm trying to say, but I hate mobile phones.... Anyway, look into the Tai Chi. If if all you do is pick a few exercises from a website and do those for a few weeks, you'll see a difference in your seat and riding.
Without the flexibility, then you end up fighting against your own body to hold a specific position. When that happens you have to remain tense to hold yourself there, and that defeats the seat being fluid and relaxed. Posted via Mobile Device