Mouth breathing is another no-no, for similar reasons to the ones that make lip licking a bad idea. Any moisture that manages to make it to your lips will quickly be whisked away if you constantly blow air across them.
So keep your mouth shut and let your nose take the reins -- that's one of the reasons it's there, after all. If you're still not sold on the idea, consider that your nose filters impurities out of incoming air and turns the air warm and moist, which makes it easier for the lungs to process it. Breathing through your mouth can also lead to things like dry mouth, sleep apnea and bad breath.