I tried a few things with all of mine. The rubbing the belly is a good dominance (hate that word) exercise. It's also a really good time to teach the sit/down/stay commands. They have to settle in order to concentrate on the commands, and it breaks the momentum they're building up. When they get a little older and just get out of hand, I use the water bottle. (Just had to use it a minute ago as playing started to get out of hand. They are now still rolling around, but more gently.)
Good Luck living through puppyhood!!
