Romeo mouthed ALOT and my hands had bruises while I figured out what worked best to train him to stop.
I took a 2 pronged approach to mouthing which I found worked.
1. When the pup mouths, say "ouch!" or whatever word you want, then get up and walk away. Don't make eye contact, go somewhere where the pup can't get to you. Sit on the couch and ignore the dog. Count to 10 then go back to the pup again and play. If the pup mouths you again, repeat! It's horribly tedious but I found it was necessary to return to the pup because they figure out what exactly it is that's making you go away alot faster. If you get up and go and never return, and only start play again hours later, the pup's going to have a hard time pin pointing why you walked away.
2. Praise when the pup is playing with appropriate toys. Because they are teething they WILL need to chew SOMETHING. But pups aren't born knowing that it's ok to chew on toys but not your hands. So, while you teach your pup not to chew on your hand (see 1 above), you have to tell the pup what is appropriate to chew. Hence, praise whenever you see the pup playing with a toy on its own.
Hope that helps!