A few things resonated with me from Mary Wanless' book, "The Natural Rider", particularly the parts about the rider's seat. I have personally struggled with my seat because my pelvis is naturally tilted forward, causing tension in my lower back. When sitting in a straight, deep position my knees will creep up, and when my thighs/knees are in proper alignment I end up perching and hurting my lower back. To find a balance between the two has taken a lot of practice. An osteopath told me that my hamstrings are overly flexible and do not "hold" my pelvis in place, allowing it to tilt forward.
To be a skillful rider you must be aware of every part of your body and every part of your horse's body, and how each affects the other. Some of the common phrases used by dressage masters include: "advance your waist", "lead the horse with your seat", "engage your seat", etc....The difficulty lies in the application of these concepts. To keep your seat "engaged" while the horse is moving at a very large trot is difficult unless you have the balance and correct muscle movement. My horse, Douwe, has a very bouncy, trampoline-like trot that I have found difficult to sit. When I would try too hard he would lose his impulsion and it became difficult to keep him forward.
Mary Wanless skillfully describes how the muscles in the rider's leg, seat, and back interplay to hold the seat in the proper place. Understanding your anatomy, and your horse's, will greatly enhance your horseback riding performance.