I have the video (on DVD): Pilates for the Dressage rider. If you use your core correctly it will whoop your butt! Plus there are some handy exercises to do while riding that will improve your seat and balance.

An enthusiastic recommendation for Eckart Meyner's writings. He has a new book out, Rider Fitness: Body and Brain, 180 Anytime, Anywhere Exercises, but you might be able to find some of his earlier stuff on the net. I had the chance to work with him once in Germany--5 minutes into my assessment ride, he asked, "are you blind in your right eye?" which I am. He could tell because of the way I carried my head--which affected the way my shoulders were placed, which affected how I was sitting in the saddle, which was why I had a tendency to sit slightly off-center and had problems with my leg position on one side. He was soooooo helpful in suggesting exercises to get my muscles relax and to learn when I was symmetrical; he's also great on increasing flexibility, especially in hips, and spine. Really good low impact stretches that you can do sitting on a chair at home as well as on a horse, using a lot of pilates for increasing co-ordination and lengthening muscles that get short from bad posture, etc. He is worth every penny