Lick your lips, press your tongue against the roof of your mouth, do a nose-yawn where you force your mouth shut and 'yawn' through your nose. Lots of things to try (not all together though  )

Yawning is caused by the brain not getting enough oxygen. The yawn brings a lot of air into the lungs. We associate it with being tired and needing a rest, our brains way of restoring itself. If you yawn a lot and are getting plenty of sleep then you may want to check with your doctor. There may be something up or you may be deficient in vitamins and minerals like iron.