We give it a time limit of sorts -- we'll tell dd that we need to brush her teeth until Mommy/Daddy counts to 20 and then we're done (supposed to be 30 seconds, but we count slowly). When that gets old, I'll tell her we have to brush her teeth while Mommy sings a song (usually we let her pick, but it can just be whatever song your LO likes). Most recently, we've been "Dora counting" -- otherwise known as counting in Spanish. Whatever works, right?