I had been riding horses for most of my life when. At the age of 26, I began reading books about riding, training, horse psychology, and many other horse related subject matter. I learned more from reading these books at my local library than I had learned from all my years of just riding. I am now nearly fifty years old and I still read every book I can about horses. I've become a good horsewoman in these last 24 years and I know as long as I keep reading I'll always improve!
Some great ideas here. Here in the UK we have the Riding for the Disabled Association, which is always looking for volunteers. http://www.rda.org.uk/