@Matt, I agree completely with the "I have a foregrip, but I think I'll hold the magwell instead". Just use the damn thing!
Anyway, right hand on the pistol grip, left hand on the handguard supporting the weight. Oh, and arms down by my hips, so I can run forwards and shoot like Rambo. Cos if it works in the movies, its gotta work in real life, right?
Actually, unless I'm firing from a static position and using the rear grip (left arm folded infront of the chest, hand firmly on the grip), I have it shouldered as SA80 types are meant to be ("the proper way"). Finger is always hovering above the safety bar, ready for the moment I actually need to go from 'boring' to 'shooty'. Will admit that for comfort or practical reasons my elbow moves around a fair bit, just because after 8 hours, 15lbs of steel can get a bit heavy.