I recently read the book "The Natural Rider" by Mary Wanless. This book was published in 1987 so it is one of her originals. I found that quite a few things resonated with me dealing with the rider's seat. I have always struggled with the feeling in my seat because my pelvis is naturally tilted very forward, causing a lot of stress in the lower back. When I would have my pelvis sitting in a straight, deep position my knees wanted to come up and when I had my thighs/knees in the right position I felt like I was perching and hurting my lower back. To find a balance between the two has taken a lot of practice. I have been to many chiropractors, massage therapists, and others over the years that have all commented on my tipped pelvis. An osteopath told me that my hamstrings are very stretchy and thus do not "hold" my pelvis in place, allowing it to slip forward. After a very debilitating fall from a horse 2 years ago I have discovered a lot about my body through all the different therapies. To be a skillful rider you must be aware of every part of your body and your horse. Understanding your anatomy will greatly enhance your performance. For so many years I just "tried" harder even though my lower back felt like a rock, no doubt it felt like a rock to my horses who would also be a little tight in the back. But I never wanted to ride with a "loose" seat either, with everything relaxed and wiggly. That is extreme in the other direction, so the key is to find a balance. Mary Wanless had some very good descriptions of how the muscles in the leg/seat/back interplay between each other to hold the rider's seat in the proper place. Some of the common phrases used by dressage masters include: "advance your waist", "take your belt buckle to the horse's ears", "lead the horse with your seat", "engage your seat", etc. The difficulty lies in the actual doing 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.