I try not to focus on it or think about it. And when I'm near gagging, I just steel myself to hold out a tiiiny bit longer. A tiny bit longer each time eventually adds up, and I've trained myself bit by bit to do away with it. I'm no pro, but I'm getting there!
Also, if I'm super turned on, my gag reflex disappears. My guy and I love to do deep throating while he's using a toy on me .. it's almost like a MMF threesome, and I can hold him against the back of my throat indefinitely (well, except for stopping to breathe!) with no problems.