indeed, #1 most common mistake people make.
If you have trouble use a breaker bar, if you do not own a breaker bar make a cheater bar, or more commonly known as a pipe wrench. Put your wrench in a pipe for extra leverage, lol.
Oh, I forgot to mention be carefull if the filler plug is off the gear oil will shoot out with suprising speed. If the oil is hot put he filler on. That way it wont shoot in your eye.