There are several ways you can control mouthing. Once you understand the following steps, you'll find mouthing will become a smaller and smaller problem every day.
Step 1: Avoid encouraging it
Have you been playing with your puppy roughly, wrestling or laughing when he mouths? Have you looked down and chatted with him happily when he mouthed because you thought it was cute or playful? Do you use fast hand motions around his head when you pet him? If you answered "yes" to any of these questions, you've been encouraging his behavior.
It's only fair that before you start asking your puppy to control himself, you stop encouraging his behavior. So, stop all rough games that involve him mouthing and stop laughing at him when he has his mouth on you. Pet him with slow strokes in the direction his fur lies. Slow movements slow him down. Fast movements speed him up. Doing these few things are excellent first steps.
Step 2: Actively encourage replacement behavior
A replacement behavior is a "good" behavior that overrides the previous "bad" one, in this case, mouthing. For example: If your pup is carrying a toy in his mouth then he cannot mouth you at the same time. When you want to interact with him, find a toy. Call your puppy over and move the toy back and forth rapidly. Get him interested in it. Give it to him and while it is in his mouth, praise him and tell him how great he is. The instant the toy is dropped, stop praising and petting him. Become neutral for a few seconds, and then encourage him to take the toy again. This makes it very clear to your pup how he can get your attention and affection – have a toy in his mouth!
Step 3: Calmly discourage it
Discouraging an unwanted behavior does not involve being nasty to your puppy. It involves making sure not to confuse your puppy in any way. For example: You're playing with your pup. He mouths you. You protest but continue to play. With pups, actions speak louder than words. So, next time you're playing and he mouths, give a quick, high-pitched yelp (a sound he'll understand) then get up and walk away. This ends the game. That's a consequence a playful puppy can comprehend. Do this consistently and he'll start to be more careful.
If your pup continues to mouth or if at any time you feel concerned that this might not be simple mouthing, ask your veterinarian to recommend a good trainer. In most cases, these problems can be quickly resolved with a little professional help. Now, go play with your puppy!