Try to position yourself so that you are always just slightly to the left of the middle of all possible angles your opponent can hit, assuming you are a right hander. This will give you the best chance of being able to reach the ball no matter where he hits it. You stand a little to the left of the middle of all angles because you can reach further to the right (on your forehand) than on your backhand, and because your forehand is hit on the right hand side of your body, while your backhand is hit in front of your body. See my article on base positioning for a more in-depth explanation of this concept, along with explanatory diagrams.Your shoulders should be facing square to where the ball is coming from, and your feet should be as far apart as you can comfortably put them – about one and a half times your shoulder width is a good rule of thumb. Place your feet also facing the ball, and then move your right foot about six inches to a foot further back than the left foot, and you will have your basic ready position. The right foot is placed further back to allow you to transfer your body weight slightly backwards and forwards when hitting your forehand. If you have your feet too square you will not be able to hit with full power. Having the right foot further back will not affect your backhand much, since there is not much weight transfer on the backhand stroke.
Get on the balls of your feet – not your toes and not your heels. Too much on your toes and you’ll tend to overbalance forwards, and too much on your heels and you’ll tend to lean backwards too much. Make sure your knees are bent, and you should have a slight crouch and a little amount of forward lean. Keep your feet light – some players like to bounce from one foot to another, and others like to jump on both feet at once. Use whichever you like, but keep moving – it’s easier to get around if you are not standing still.
For balls that are table height or lower, bend your knees more to get down to the ball, rather than bending from the waist. Using your knees allows you to hit the ball using your normal technique, while bending from the waist changes the way you have to swing, since your body is now leaning over. Try both ways and you will see what I mean.
Keep your crouching position as much as you can while playing – this helps keep your center of gravity low, and will help you move around the court more smoothly. Standing up straight will raise your center of gravity, lock your knees and impair your ability to balance and move quickly.