I hold the magazine well and my left elbow rests on my ribs for a very stable and comfortable hold.
I use this type of holder to not put a pull on the mag itself which I believe might cause misfeeds
http://www.amazon.com/dp/B004LOEBJM/...SIN=B004LOEBJM
I tried to like a foregrip type but found I couldnt hold for long periods of time.
After trying mine all 3 of my AR shooting buddies switched to it