Hi,
well, if you go a little down the list (to a post made by me on the 14th of October) - something about strengthening your back.... you will find some good exercises.
Also, consider Yoga, TaiChi - or what has become very fashionable lately: Pilates.
Anything that helps you to become more aware, more balanced, more co-ordinated, more symmetric, stronger, fitter, more flexible and improves your mind/body connection.
Another useful "mind exercise" is to try and see things from the horse's perspective. Practise to feel the intensity of your aids and assess if you could be lighter or more tactful to be understood (you can play this with another person.. they will protest very quickly if you are too rough..
..... start with patting! (are you a "stroker" or a "slabber"?)
Walking certain movements, like all the lateral movements can also be quite enlightening and you may find that walking a shoulder-in for example is a reflection of how you would ride it on the horse....
Have you tried one of those Gymnastic balls? (I bought one but haven't really tested it yet (laughing too much - and then I fall off, hehe...)
Rowing (legs, back and abs)
Swimming
Have fun!