We used a trick on our weimaraner puppies which we learnt from the breeder she trains her dogs in obedience and field trials where a soft mouth is important on a retiever breed. She treats her dogs like (junior) family members and expects nothing less from the people who have her puppies, she also expect you to raise them properly, so as not to bring the breed into disrepute through out of control dogs.NB ONLY DO THIS IF YOU HAVE SHORT FINGERNAILS...and don't try it on full grown dogs. Let the puppy nibble on your fingers, as they bite down tickle the roof of the mouth gently and say to the dog "gently" The dog should open his or her mouth and look surprised..Praise lavishly. As the pixie says, never allow the dog to bite down or chew things that you won't want it to later. The reason to let the dog mouth your fingers is so that you can control the mouthing. if you let it chew on clothing, it will think that the clothing is part of you and the chewing doesn't hurt and not learn not to do it. Later on, teething pups will need something to teeth on and will try to chew on any thing they can if not prevented and given something of their own. If the mouth play gets too rough with an older puppy, don't shout at the dog, but yelp like a litter mate would and stop the game. Where did you get the dog from and will they help you? also the battersea dogs' Home website is full of useful stuff.
http://www.dogshome.org