I've been riding three or four times a week and on the off days using different free yoga, stretch, and exercise videos from DRESSAGE RIDER TRAINING. It's kind of amazing to me how working a little three or four days a week on my flexibility and core strength has improved my riding quite a bit. Really quickly. Because I am more balanced and have more body control everything feels so effortless.
Another interesting thing has been reading the posts on the DRT BLOG to understand the physiological underpinning of how dressage riders should function physically. This is all stuff you've maybe heard before from Mary Wanless or Sylvia Loch but explained MUCH more concisely(think four or five paragraphs) with video and written explanations, so you can do it right then and there. The beauty of this site too, is that you can then go look up exercises that will help you be stronger or more flexible wherever you need to improve.
There are a lot of blog posts I have found useful but three especially stand out to explain the why. STRENGTH FOR DRESSAGE explains the type of strength necessary for dressage, so the rider can be aligned and with the horse, yet in lightness. NEUTRAL SPINE goes into how to use the whole core to brace and protect the spine, yet easily follow the horse's movement. RELAX delves into whether relaxation means being a blob or having the right muscles engaged, so the rider can stay aligned and in balance but have other muscles released, so there is an air of lightness.
Nothing on this site tells you how to train a horse. It is all about making sure the rider is more physically correct and using the right muscles to stay in alignment and straightness much more easily. Check it out and tell me what you think.