to fall out of the wall, try this: invert the wall receptacle so the ground pin is on the top. That way the strongest grip is at the top of the plug doing the most good. Even if the weight of the cable is trying to work with gravity (there is that physics thing again) the blade contacts will retain more of the blade in the receptacle.
I keep forgetting to post simple things like this. Hopefullly you are all ahead of me here.