For me its about stance and attitude.  Mine not his.  I stand up and keep relaxed but get in that Marine stance(no fear) and say no loudly whenever he doesn't do what he should and then when he stops, I pet him and tell him good boy.  If I have to I back him up a little to let him know he has not dominated me.  Both of my dogs know that I mean business once I stand up in front of them.   Just don't forget the praise when he does well.  You don't want to stand up to a dog that is already fighting or a strange dog though.  That can be very dangerous.