Stripped Oil Drain Plug

I am very frustrated. The oil drain plug on my car was in tough shape and was put on very very tight by someone! Now I have made the situation worse, stripping the bolt even worse trying to get it off. The bolt is now somewhat rounded and it won't move. Does any body have any hints how to loosen it now that the bolt head is worn. (How can such an easy project to lead to so much frustration?).

Bill