I would suggest clicker training. I just started doing it and I wish I'd gotten into it sooner.
Since you can't lure your dog into the sitting position (since he's already sitting)...then maybe this would work: If you used a clicker (or you can just use a particular word such as "Yes!"), you can just sit and observe him while he's playing or whatever, and any time he does what you want (such as sitting), click and give him a treat. After awhile, you can tack on the command and say "Sit" as he begins to sit, then click and treat. Finally, after he gets used to the word, try telling him to sit first, then if he does, click and treat. If he doesn't, go back and work on the last step.
I like this way much better than trying to use any type of force or pressure on the dog to make them do what it is you want them to do.