This is why it's vital to teach a puppy not to bite so that it doesn't become a habit, and if he is taken by surprise as an adult then he inhibits himself from biting. Therefore, for the youngster's own good, you must be prepared to play act like an Oscar winner. When the puppy's teeth contact your skin, be prepared to yelp and look hurt, indeed if you can sob and force out a few tears, so much the better. The more convincing your 'performance' the better the pup learns the lesson.
Learning not to bite should be taught from as young an age as possible, indeed, puppies playing in a litter start teaching each other from just a few weeks of age. The principle is that when a puppy bites too hard, his littermate squeals and the game stops. Thus the puppy learns to be soft and 'play bite' so that everyone is happy and the game carries on.
We can mimic this lesson by yelping whenever the Lab puppy's teeth contact skin, and by withdrawing from the game if the pup doesn't immediately calm down.
Instead, it's best to yelp and let your hand go limp, so the puppy understands he hurt you and then the game stopped.
It's also helpful to have: