Start by having someone fire wrenches at you and see how your reaction time quadruples. "If you can dodge a wrench, you can dodge a ball". Sorry. Couldn't resist that line from the movie "Dodge Ball".
Seriously now, I think that anticipation is the key ingredient to reacting to a shot and anticipation can be enhanced by paying close to attention to how your opponent plays certain shots. Perhaps you notice that your opponent almost always throws up a lob off a good approach to their backhand. Knowing this, you can anticipate and be ready to retreat to hit an overhead. Obviously that's just one example, but it applies pretty much accross the board.