At that age I just gave DD a toothbrush and let her chew on it. While she was chewing I would brush with a second toothbrush - the first kind of acting like a wedge to keep her jaws open (lol).
Now that she is a bit older I have had GREAT luck with a tip I picked up from another parenting forum. I let her "brush" for a minute and then I ask to see "if Elmo did a good job." (He's on the toothbrush). I then gasp and proclaim that Elmo got the monkeys (etc) but "omg he missed a ZEBRA! I have to get the zebra!" With lots of exaggerated gestures and such. She laughs and will open her mouth quite happily!