Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
If you dodge directly backwards when you're being shot at you'll probably still get hit.
Time your blocks well to negate all of the damage.
If an enemy has a sword, stay back and shoot. If it has a gun, get up close and cut.