Improve your reaction time on return of serve

Volley-to-volley drills are good for improving reaction times generally, but here's a good practice to help you improve your reaction times on return of serve:
Practise returning a friend's serve, but simulate a much faster serve by getting him to serve from the service line instead of the baseline. He'll need to place the ball in front of him and use a lot of wrist snap to avoid hitting the ball long, but he should get the knack after a few attempts. As for you - you may need to adopt a neutral grip (halfway between your forehand and backhand grips), you may have to adjust your waiting position and you may have to use an abbreviated swing. Don't play the point out. Return half a dozen from the deuce court, then half a dozen from the ad court, then the two of you can change roles. After a while, you can make it competitive by counting how many of the 12 returns land in court and your friend can try and beat your score.
The abbreviated swing is achieved by placing your elbows close in to the side of your hips instead of out in front of you. That way you'll shorten the radius of your backswing, making it faster.