I have only been using CPAP for about a month. In the beginning, I would wake up quite a bit with air coming out my mouth, or with my mouth full of air, but closed (bet I looked like a puffer-fish!). That seems to happen less as time goes on. Now I might wake up once during the night due to opening my mouth, if that. That also reflects in my daily AHI, which went from an average of 6.6 in the first 19 days, to now being under 2 every night for the past week and a half.
I also notice that on nights when I don't wake up, my tongue is plastered to my front teeth when I wake up. So I agree with Daniel. Make a conscious effort to keep you tongue from falling down and back, and that will gradually get better. Now that Daniel mentioned it, I realize I do place my tongue on the roof of my mouth toward my teeth to go to sleep!
Diagnosed September, 2010
ResMed S9 Elite, Pressure 10
Zest Nasal Mask