I use the free toothbrushes I get bundled with toothpaste. I put doggie toothpaste on them and hold the brush to make sure he gets all his teeth. He just chews away! He will try to run away with it so I hold it. Also, I just got him one of these:
www.easybrush.com It's thick though, so I'd only recommend it if you have a bigger yorkie (with a bigger mouth!) Loki is 9lbs.