During my time walking dogs at the local shelter I have come across numerous dogs, no longer puppies, who mouth non-stop the moment you enter their cage. Mouthing/play biting is something that puppies naturally do to play and ease the pain of teething, but if done on the wrong items (i.e. your hand or wallet) it can be very painful and destructive. Puppies who are allowed to mouthed, or even encouraged to play this way will never grow out of it, and you are left with a dog who gets excited and starts biting your hand or clothing. I've seen this with many pit bulls. I assume that their owners encouraged them to mouth when playing, or did not correct it, and the dog has grown to be an extremely intimidating dog who bites your hand when being leashed, jumps on you and grabs onto any item of clothing or skin it can get a hold of, and does not let go. This can be very scary for anyone, no matter what their experience level is.
The key to preventing this behavior is catching it while the puppy is young. Here is a very good video that shows a technique you can use (if you don't mind sounding a little weird to people who don't know what you are doing).
As stated in the video, this method is only useful when a puppy is still very young. So how do you stop mouthing with an older dog? Ignore, redirect, or discourage.
When a dog starts to mouth you, you have three options, and you can use either one depending on the situation. If the dog is trying to play with you or get your attention the best thing to do is ignore your dog. Turn away from him and wait patiently until he has stopped and is ideally sitting quietly, then give him what he wants. This is also a good technique for if your dog mouths while being leashed. But be warned, this can take a lot of time and patience to achieve.
If you are already playing with your dog, or happen to have a toy at hand, you can use that to redirect your dog to an appropriate thing to mouth, and praise him when he puts his teeth on it instead of your ski
Last would be use discouragement to get your dog to stop mouthing. If your dog is being dangerous, like biting and not letting go, I would use this before anything else. But other than the most extreme cases this should be a last resort, You can use citrus spray, compressed air, a loud noise, spray bottle etc. to correct and discourage your dog from biting.
Other things to keep in mind is to not rough house with your dog, as this encourages using the mouth to play. Having your dog on a leash can also help you correct him every time he goes to bite you. No matter which method you choose, make sure you do not unintentionally turn it into a game, for example pulling on the leash to get it out of your dogs mouth turns the struggle into an encouraging game of tug-of-war for your dog.
And here is a dog salsa dancing...why not?