Teresa Ford's ideas are excellent. Too bad you can't get into a class with her!
I have also found that holding a small treat high enough above the dog's head will actually cause him to sit as he looks up at it. If the butt does not go down as he is looking at the treat while you are giving the sit command then gently push the little butt down. "Good boy!" and give him the treat. It may take a few times before he connects the command with the action but when that nose goes up in air as he is looking at the treat the butt will go down when he figures out that is how he gets the treat!
