Always breathe through the nose rather than the mouth: the nose acts as a filter to potentially harmful airborne particles and bacteria. Mouth breathing dries the gums and leads to bad breath. Remind yourself to shut your mouth every time you look at your watch or save at the computer. Exercises can help. Pay attention to where your tongue lies naturally: it should sit against the roof of the mouth at rest and when swallowing. See also that your upper lip drops heavily onto your upper gums, weighting down the teeth. If you suspect age-related drooping of cartilage in your nose is affecting your breathing, visit your doctor.