Buddy is really quick with the nip. He's only 3 months old now and I want to train him before he gets quicker and bigger. Since he's so little he can nip and run before I can catch him so what I've found to be effective is, when he gets in the nip mood (usually early morning and later evening) I clip a leash on him so if he starts to run, I can easily step on the leash and stop him. Then I close his muzzle and say No Bite!
I have found the leash to be really helpful since he now realizes he can't get away with it. Often just putting the leash on him makes him stop. I've also realized that he often is just trying to get attention so if I haven't exercised him lately I do that too. The old Exercise, Discipline, Affection (in that order) regime.
