Personally, I like Cesar Milan's rule - clear, concise, nothing conditional: dog teeth do not belong on human skin. <-- period. You might taste good (salty), you might think no 'harm' is being done because it doesn't hurt, because it's 'just' mouthing, because the dog's not angry, etc - but those are all your conditions and the dog doesn't necessarily get it.
I've always stopped this by over-reacting whenever dog teeth contact MY human skin (loud "OUCH OUCH" or whatever you choose.) Does the trick quite succinctly. =)
The rule I've always had with any dog I've ever been around is their teeth can contact my hands ONLY, and if and only if I initiate the game, and they must stop instantly the second I say "enough."
I've never had a problem, but it does depend on the dog. They are certainly capable of grasping the idea of rules and boundaries as to when and how it's appropriate; dogs play with other dogs by biting all the time, yet I have yet to see a well-socialized dog just go up to another dog and start chomping on it as, say, part of the greeting ritual.