I've noticed alot of people here use the little finger toothbrushes. Riley's mouth is still too small for one of those. If your puppy's mouth is really small also, I would recommend this toothbrush from dog.com. It has a big end and a small end. The smaller end is smaller than a child's toothbrush and it works great. They might sell these at pet stores. I havent looked though.
http://www.dog.com/itemdy00.asp?T1=311310%20001