Just keep trying masks. I went through several before finding one that seemed to fit right.
As far as "doing something wrong," I discovered two things. One, I reduced the leakage by loosening the mask strap--just barely enough tension to hold it on my face before entering the water. The other thing I discovered is that I am prone to unconsciously making facial expressions that distort my face just enough allow a little water to seep in. For example, if I see something that makes me think "wow," I might grin. As weird as it sounds, keeping a bit of a poker face has reduced the leakage for me. The more obvious culprits under the "doing something wrong" umbrella include unconsciously breathing through your nose rather than mouth, but I suppose you're aware of that one.