I resently had the pleasure of working with a 6mth old standard poodle with a nipping problem. Two things before I continue.
1. He nipped non-agressively.
2. I am not a professional trainer :)
When I first met "Dekota" I was greeted with a firm chump to my thigh. He nipped trying to illilicite play, but hurt as he did so. His owner who had tried leash-popping and yelling, had only succeeded in exciting him more. She was down to the last straw and asked if I could help. So I took him in for awhile to work with him.
First I tried "Yelping" when he bit. This only made the problem worse, he got very excited at the sound of my yelp. I then tried walking away from him, but he decided this was also enjoyable.
I started working on some (clicker) obedience. He learned to shake and sit within 4 sessions. I then taught him to lie down. You may wonder why I did this instead of working on the biting directly. One reason was that I wanted him to get a firm understanding of what the clicker meant. (The shake was thrown in because he kept offering it.) The second reason is that I have had experience with dogs that could learn a few cues and would offer the cued behavior rather then nip. Which was not the case with Dekota.
When none of this worked I began tackling the biting with the clicker. I told him to sit and when he did I waved my hand around his face. He nipped once or twice at it, when he turned his head away I clicked. He quickly caught on and would not bite when the clicker was present. But when it wasn't in site he would start nipping again. So I started playing the "Magnet game" with him. When he nipped, snapped or mouthed he was completely ignored (he is the kind that you can't even give a no reward marker to because he considers it attention.) When he behaved I would talk calmly to him and pet him. If he tried to nip when I went to pet him I would turn sideways and ignore him (I never turned my back to him because he enjoyed biting tushes I quickly discovered).
After consistantly doing this over a few days I was no longer bitten! and neither were his owners or anyone else he encountered.