Mayo, in my experience, having sold them for over a year in a dive shop, most new mask leaks are because you have a narrow face and a wide mask silicon cradle can't seal at the temples, or you have a very wide face and the mask is not wide enough to clear your eyes, or, you have "happy" lines (facial wrinkles) and or a mustache and it leaks from your cheek area.
Solution: if you have a narrow face, try a junior mask. If wide, get an extra wide mask (the technisub Ventura is good). The happy line effect, I find, has been made worse by too/very flexible silicon cradles and I wish they still used rubber. The Cressi "Big Eyes" mask and similar "improvements" on visibility often leak from this.
A few people have nose/brow configurations too big for the nose pocket to sit properly and therefore they leak all around or they must be strapped tight and hurt, not a good idea or comfortable.
As to fitting: What you should do is, over a bench or cushion or soft chair (so if the mask drops you don't break the glass), looking straight ahead, place the mask on your face without the straps on, and lightly push down on the frame, attempting to make a vacuum seal. You will know if there is a seal becuase the mask will "stick" to your face for a second without holding it. As was said, do not breathe in to assist. If you get at least a second of suction before the mask releases then light tension from the straps should keep it leak free.
I've been snorkeling and diving for 35 years and my only regret is losing my 22 year old Dacor rubber mask, which was still good and leak free when I dropped it assisting a diver with free-flow.
So last tip: get a strap with a foam/cushion backing that floats!!!
