I've been using a squirt bottle to train Socrates. We've been going round and round about his biting. He just wants to play but I am trying to teach him that biting is a bad thing.
When he starts biting I'll tell him "NO!" and sometimes that will work. If he continues after the second "NO!", I say "I'm gonna get the squirt bottle." That works now as he's experienced the squirt bottle enough times to know exactly what that phrase means. I don't even need to squirt him any more, just threaten to get the squirt bottle and he immediately straightens up.