Answer:
1. Practice Blocking Drills, where your partner throws the same combination over and over. Do 25 Reps Slow Speed, 25 Reps Medium Speed, 25 Reps High Speed. Repeat for different Combinations. You can use padded foam blocking sticks.
2. Train with a reflex bad, or double ended bag.
3. Purchase a reflex ball, and throw it against a wall. Repeat over and over.
4. Work on your foot work, and head movement. Try the above #1 without blocking, but rather dodging.
5. Purchase a Neuro Bio Feedback game for your computer. Sounds strange, but what it does is train your brain to react. It will also help concentration which is important for reaction (hard to react when you are not paying attention).
Off the subject I recommend shadow boxing while holding a marker or something similar in size under your chin, to make having your chin down a habit.