Whether you're about to return a drive serve, or you're in the middle of the rally getting ready for your opponent to hit, it will benefit you greatly if you put yourself in the "down and ready" position.  Your feet should be more than shoulder width apart, your knees bent so you can spring into action, and your racquet in the backhand grip (because most people will hit to your backhand, and it's an easy switch if they go to your forehand anyway).

 

If you're returning a drive serve, you should be facing the front wall of course, and watching the ball as your opponent hits the serve.  During a rally, your feet and body should be facing the front wall, but you should turn your head slightly toward your opponent (depending on which side of the court they are on) in order to see them hit.  One of the biggest mistakes I see is people not watching their opponent hit their shot.  They get down and ready while the ball travels to the back court, but they face the front court and wait for the ball to come off the front wall.  BIG MISTAKE.  You want to turn and see what shot your opponent is going to hit, get an early read on it, as well as protect your behind if the ball you hit should bounce behind you in center court.  A lot of people get hit in the butt when they don't watch their opponent hit.

 

Being down and ready gives you the best opportunity to move to your left or right as quickly as possible.  Think of yourself as a lion, ready to leap at your prey.  You don't want to shade yourself to the right or left, because you'll just make yourself more vulnerable to the opposite side.  I see a lot of players shading toward the backhand side, thinking I'm going to hit the ball there.  A nice rocket cross-court usually teaches them a quick lesson.

 

In the following clip, you'll see the server fail to get in the down and ready position after his serve, and as a result his timing is off by just a fraction of a second.  It causes a lapse in his footwork and swing mechanics, leading to a missed pinch (literally by inches), that gives his opponent a set up.  Small changes like being down and ready can take your game to the next level.