Once in a while I still THINK I exhale occasionally through the nose. Not sure, but it seems I clear my mask more often on a dive once in a while. I like the advices given on practising this & that in water/snorkelling, etc. I would imagine you could also practise breathing this way and that without the water as well. Closing off the nose, breathing only from the mouth. Vice versa. Mix it up. I have found that normally I am a "both" breather, inhaling & exhaling by nose & mouth--sometimes separately, sometimes at the same time. Getting control of which place you want to breathe from is the answer. When diving think "my mouth is my only source of air". Get it so you think of the nose as non-existant. I once helped a student who couldn't fully clear her mask. She was a mouth breather, so when she tried to clear the mask, some air also came out of her mouth. Another example of practising what you want to do and getting those muscles used to it.