get into the habit of rolling... PERIOD. don't just stand there and hit the bag. Hit the bag and move... hit it and roll to the left, then to the right, etc. Hit it and move your head. Hit and retreat. Etc.

Becoming a better defensive fighter and being harder to hit is just as much a function (if not more) of you learning to slip/bob/weave/roll/whatever BEFORE a shot is thrown at you as it is you REACTING to the shot being thrown at you. You are NOT faster than a punch. But you are faster if you move first.

So it isn't just, he punches, you move... it's also, you move first, he punches, but he misses, you counter.