If you touch your tongue while yawning it can stop the yawn

If you touch your tongue while yawning, it can stop the yawn mainly because your tongue is the most sensitive thing on your body.

Lick your lips whenever you feel a yawn coming on.
Seriously, it works. Now you'll be needing some practice to be able to this when it matters. You won't automatically lick your lips when you feel like yawning.
We know that yawning is contagious. So for practice, I'm giving you a link to a yawning video. Watch the video and whenever you feel like yawning, start licking your lips.
http://www.dontyawngame.com/