The only thing that's worked for me is to just try to keep as relaxed as I can. When things get more intense, it becomes much harder to keep my gag reflex quiet. For example, if my partner is thrusting quickly, I find it harder to deal with, but when I'm just going along at my own pace I can get an awful lot down my throat. All I can say is relax and practice, really. I wish I had some better advice
I find a bit factor for me (cause I've totally been struggling with the problem lately for some reason!) is to make sure my neck's at a good, comfy angle, because if my body is in tension there's no way I can relax enough for deep throating.
I haven't experimented enough with this to really know how well it works but it seems to be helping.
Whenever you brush your teeth at end of brushing, brush your tongue from the tip to as far back as you can without gagging too much. I've heard that eventually you'll be able to brush farther and farther (eventually to the point where you can touch the brush to your uvula without much/any discomfort)until your gag reflex is pretty much gone.