I've also found that hand gestures work wonders too. I had that problem with cora, she's bitey when she plays and that's fine, but when I'm not in the play mood, i don't want her nipping at me, so I try to be really consistent to say "Play" when its ok to play, and I hold my hands up (like enough, palms out sort of deal) and say "no more" when play time is over. She's taken to it really well and knows when to stop now.