Here we go:
-When you are looking at an OTTB, soundness is a real issue, especially with respect to leg/tendon/ligament damage. Vet checks are great but if you can't have a vet look at the horse, take someone experienced with you to look for soundness issues, old injuries and conformational advice.
-Once they have finished racing, turn them out in a good pasture and give them a few months 'let down' period. They are fed some pretty potent supplements so it is always a good idea to let everything work it's way out of their system before you begin working with them. The key is good quality hay and roughage and plenty of it. Also, being at the track can be a very high stress environment for a horse so a bit of time off is always a good thing.
-When you bring them back into work, treat them like a green broke horse, sure they are fine to ride but they need to be completely re-educated to establish three even, balanced gaits. If it takes a month, great that is really quick, if it takes six months, don't worry the extra time spent in the beginning will serve you well in the future.
-Don't rush their training, just because they pick things up quickly doesn't necessarily mean that the basics are firmly entrenched in their mind.
-Get their teeth checked. Although some trainers are very good about regular dentistry they are few and far between. Most OTTB's will be 4-8 years old when they finish racing and many have never seen a dentist in their whole life.
-Introduce new diets slowly and be ready for them to drop some weight when they come off the track. A racehorse diet contains far more calories than what the average horse is fed, again good quality roughage is key.
-Before you take them out to compete, take them to a show without entering them in any events and see how they handle the atmosphere. Most OTTB's think they are back at the track when they see all the horses, trailers and commotion so taking them out without the stress of competing is a good way to ease them into competition life.
-Patience patience patience!
Anyone care to add more? I will probably add more things as I think of them