Check it with a snorkel or something in your mouth. Nobody mentioned it before and I found out after it was too late for me. When you put a reg in your mouth it can shape the shape of your face a little bit. My first mask fit perfectly in the store, according to every method I read for checking fit. I tried a BUNCH of masks and bought the one the fit the best, even though it was the most expensive (i.e. more than I had budgeted).
Then I found that with a reg in my mouth I got some wrinkles (smile lines?) that made it leak just enough to be clearing my mask every 2 or 3 minutes. I attributed the leakage to facial hair until I tried a buddy's mask to see how the magenta lens was and just happened to notice that I never had to clear my mask a single time during that dive. I subsequently bought the same mask as my buddy and have been (nearly) leak free since.
