If I told you you'd be riding better with very little effort and no money involved, wouldn't you go for it?
I had the privilege this month of working with some riders participating in a clinic with U.S. Olympian Steffen Peters. Aside from some videos I have seen of Reiner Klimke, I don't think I've observed many riders with such correct posture. You may recall a recent article in Dressage Today on Mr. Peters' personal training workout (March 2009). As a trainer, I paid close attention to that article and also to his comments during the clinic. He stressed the importance of the lower abdominals for maintaining correct pelvic alignment so that your weight is balanced correctly on your seat bones, allowing you to use your seat effectively. (I'm sure you haven't missed the emphasis on half halts in recent editions of DT.)
To keep your core strong enough to sustain your posture throughout your ride, it's really important to do more than just ride. What I've noticed in my personal training practice is that it does not actually take much time to maintain a very strong core once you have it.
Building a strong core does take time, though, and needs to be done in small amounts. Because your core is, well, at your core, you build its strength and stamina from the inside out. It may take you months to acquire a six-pack, but you can ramp up to noticeable improvement in just a couple of weeks. That's good news because, now that we're in full swing of the horse show season, you need to know what you can do to improve your stamina in time for the championships--when it's going to really count.
The rider in the photo here is a young client of mine who is competing to get listed to the Junior Rider team. She is a very serious rider with exceptional technical skill. But, she is only 15 years old, and photos from my assessment of her ride provided a perfect illustration of what happens when a rider has weak lower abdominal muscles. You can see that her pelvis rolls forward a bit, pulling her belly a little forward. Biomechanics are causing her legs to shift forward a bit as a result, and she spends much of her ride trying very hard to get them back again. Second, her shoulders lean back to compensate for the forward tendency at her lower back and pelvis. Effectively, she has lost the secure base directly above her seat bones, which is needed to do a proper half halt or a driving aid.
You can see that the biomechanics of the situation include her horse: his nose is pointing down almost directly on the same line her shoulders have created by leaning back. She has created pull/lean balance through the reins, and her pelvis' rolling forward further drives him onto his forehand. Her legs are relatively ineffective because they are creeping up to his shoulders. (It is almost impossible to get your horse ahead of the leg when your legs are ahead of most of him.) Her horse is quite large, especially in comparison to her body size. So, when he gets tired or lazy and bears onto the forehand, she does not have the strength to maintain the self-carriage she wants him to have.
As a younger rider, she has a number of aspects to her exercise program. But, a large segment of it is devoted to lower abdominal strength, which will help her to straighten her body. She will use stronger lower abdominals to create more alignment from hip to heel, and more strength for half halts and holding her posture when her large horse wants to bear onto his forehand. Incidentally, the same exercises that build lower abdominal strength also seem to work the lower back area. I have not met a rider with weak lower abs who did not also have a weaker lower back. Doing exercises for this area, you want to feel a burning or fatiguing sensation but not sharp pain. If that occurs, cease the exercise and consult a physiotherapist or your doctor.
Unlike strength training for other areas of your body, core work can be done almost daily, as long as you do not overdo it. In fact, doing your core work at least five days a week will really accelerate your results, and it will only take a few minutes a day with no equipment.