I agree with Shellbyville...i have a GSD puppy and he was very mouthy, so i used the treat method always getting him to sit when he would greet me and i would treat. If he got impatient and went to mouth he would get a loud uh uh and i would walk away and come back a few minutes later and i would have him sit, drop and then would treat. After strictly following that he was vastly improved within a week and now he rarely mouths at all, and if he does he gives me an "oops shouldnt have done that look" and quickly sits and drops and tries to give paw (shake) to make up.
I initally tried the puppy yelp technique that dogs4fun suggested to you but it only worked the first half dozen times for me and then puppy just ignored the yelp and kept mouthing. I think the trick to this one is in perfecting the puppy yelp and i just could not master it although i would love to know what the neighbours thought was going on as i am sure you could hear me two blocks away.
Good luck and just keep trying because it will not stop without a lot of effort on your behalf....