One thing that I tell students is to think about how they are breathing in a different way. We all breath through our noses most of the time, but there are plenty of times we breathe in and out exclusively through our mouths with no problem whatsoever. By thinking about those times, I've had luck getting even the worst nose breathers to stop almost immediately:
1.) Breathe in like you're sipping hot soup from a spoon
2.) Breathe out like you are blowing out birthday candles
In both of these cases, you move air exclusively through your mouth. Folks have told me that they'll spend one or maybe two pool sessions thinking "Hot soup... birthday candles. Hot soup... birthday candles..." and then realize "Hey, I'm not breathing through my nose anymore!" and they can stop thinking about it.
1.) Breathe in like you're sipping hot soup from a spoon
2.) Breathe out like you are blowing out birthday candles
In both of these cases, you move air exclusively through your mouth. Folks have told me that they'll spend one or maybe two pool sessions thinking "Hot soup... birthday candles. Hot soup... birthday candles..." and then realize "Hey, I'm not breathing through my nose anymore!" and they can stop thinking about it.