When I bought my first OTTB I was lucky enough to work with a trainer who had a long and successful history of restarting racehorses. It made a huge difference to work with someone who understood how a racehorse had been trained, what it expected from its rider and how it had been ridden.
Today I came across a great e-book: Understanding and Retraining the Off Track Thoroughbred by Kimberly Clark. You may know Clark through Leighton Farm and Thoroughbred Placement and Rescue, which helps OTTBs transition from racing life and find non-racing homes.
I highly recommend reading it if you are considering an OTTB . . . or even if you already have one.