Sleeping with your mouth open can contribute to multiple problems. The dryness that open-mouth breathing creates within your mouth and throat can irritate the tender tissues of these areas. It also can create more optimal conditions for bacteria to grow within your mouth, which can lead to gum disease. Open-mouth sleepers also tend to snore, which can be extremely frustrating for others in your home.
Sleep on your side or stomach. Sleeping on your back can make it more likely that you will sleep with your mouth open.
Use a nasal spray or a decongestant if a stuffy nose is causing you to need to breathe through your mouth. Be wary of using these products for extended periods of time, though, as they can actually worsen stuffiness with extended use.
Try an oral vestibular shield. These shields are fitted to your mouth and help to prevent open-mouth breathing and snoring.
Use a nasal strip. These adhesive strips fit over the bridge of your nose to open nasal passages, which can reduce the need to breath through your mouth.
Use a chin strip. These devices, some of which fit around the head, keep the mouth closed during sleep. You must have at least one nasal passage open in order to use this, of course.