does it do that when vibrating? if it does, then a phone case would probably help. my phone falls off the table regularly w/o a case just by vibrating its way off.
if not, maybe an extension cord could help. i haven't had many problems w/ that since we moved last yr (b/c all our plugs are rigid) but at our last house, i remember we had to get extension cords so plugs wouldn't just fall out of the socket.
do u move the charger constantly?