I also have an OTTB. He is such a sweetie but he has had his bad moments and there are more to come. When I first got him I rode in an english saddle and then gradually moved onto a western saddle. I still use a D-ring Snaffle bit. First thing to do is let him run around outside in the pasture as much as possible. This will get most of his excess engery out. Then when you want to get him out start by only doing longing. This will make him slow down his mind set of "once I get out I am to run". This also helps with your voice commands and having him get used to you and build trust. Most TB are taught that if you cluck to them its for walking and trotting, kissing to them is for a faster pace or gallop. When you do start riding him remember that the harder you pull the faster he goes and the further you sit forward the faster he goes. Be loose on the reins and sit deep in the seat. Talk to him alot. Race horses like to be talked to. If he starts to go too fast for you sit down and seesaw the reins and tell him EASY or Easy Walk. I have also used draw reins on my TB this helps him bend and keep his nose down. Do lots of circles and keep him busy. Use leg pressure and lots of pats on the neck and words of encouragement. GOOD LUCK!
A very important part of owning an OTTB is to give them a good long time to detox. Racehorses are fed huge amounts of grain as well as "cocktails" to give them a racing edge, and all of that doesn't just go away within a week or two.
This was really helpful as just last week I was riding an OTTB in a lesson and had very little clue the best way to communicate with him (I hadn't ridden him in a while so we were out of sync). I had no clue that he had been leaning on the bit the whole time but boy oh boy I could tell when I dismounted and relaxed my shoulders.