Losing weight and not sleeping face up may help in any case.

If you usually feel tired or sleepy during the day, it could be sleep apnea, which can have long term negative effects on your health. In that case, see a doctor, who will usually perform tests while you sleep and then may prescribe CPAP. If that doesn't work or you find it too uncomfortable and the apnea is severe, you may be offered surgery.

There are some commercial devices that might help. Nasal strips are an option if you suspect your nasal passages could be compromised (deviated nasal septum). Chin straps and other devices that position your jaw do help in some cases wherein the issue is in your throat, especially when combined with CPAP.

Edit: also, don't take medical advice from people on the internet ;)