I put Nanook on a leash, and I sit on the leash while I work him. If I see him start thinking about sitting down, I put a hand under his belly, down by his groin, and say "eh eh - Stand."
He gets the same treatment if he tries to circle away, "eh eh - stand." And I don't stop until he is standing politely. He gets a cookie and released from the stand and the grooming session ONLY when he is standing politely. Sometimes in order to reward the good behavior I have to stop before he's done - I try and judge when he's really had it, and stop before he reaches his limit.