By mouthing do you mean just love bites? As in it isn't hurting but you don't like this behaviour?
How I react to playful bites that hurt: A sharp, yelp, "OW!". The first few times, they'll usually stop, then come back again to play. If by the third or fourth time they continue, the play session ends.
If you mean mouthing, as in just an unwanted behaviour, I would ignore it. My sister in law has a 4 month old pit that does the same thing. WHen it's really intense and can't be ignored, I issue a firm, sharp verbal correction, "EHT!", "WRONG!" and continue to ignore. When he sits quietly, then he gets attention. A lot of the time once you begin to pay attention to them, they'll get excited again. Do the same thing, "WRONG!" and ignore.
They figure out pretty quickly, the mouthing, nipping and jumping excitedly all over you does not get the attention they want.
You have to make it clear though what behaviour is expected and acceptable and highly reward them when they do it.
Instead of asking for them to behave for attention, don't give them attention until HE does the behaviour you want.
Ex) Your dog is mouthing you. Instead of saying, "Sit" good boy, and praising.
Say nothing. Ignore the bad behaviour, and as I mentioned above if it's intense a sharp verbal correction. When the dog does the desired behaviour on his OWN than reward. This way you get a dog that ALWAYS behaves, instead of one that waits for you to tell him to behave.
Good luck
