Teaching Your Dog to Lay on His Back
Dog's are not crazy about laying on their backs, particularly if they did not learn how to do so as a pup. It is a position that makes them more vulnerable so I think it probably stresses them unless they learned as a pup to lay that way.
So to start, this is best taught as a pup when they don't have as many fears. It can be taught to an older dog. I think the way to do it, is to teach the dog the trick "Dead Dog". They will learn to lay down and roll to their back. After all, play dead is just a partial "roll over".
To start, I am attaching a video on how to teach a dog to settle or relax.
http://www.youtube.com/watch?v=FRM0LeSBjxA
Once they trust you enough to lay down and settle, then you can teach the "roll over" or "dead dog" trick. After you break it down to those tricks, you proceed to teach them to roll on their backs and they learn they get a belly rub.
I am working on my own video of how to problem solve on teaching a dog to lay on it's back and be calm. We will post it as soon as we get it done. I will be using my Midas, who is now one year old.
Keep a watch on my blog. In the meantime, if you want to teach your dog to lay down for a belly rub, start by teaching the settle/relax command.