The game of tennis has changed a lot during the past 20 years. With advances in technology and a physical development, the same concepts that worked 20-30 years ago are not as applicable today (the emphasis being, of course, on "as" since some older concepts have limited application today).
One such concept is that the player should look to come in on a short ball. Sometimes, this is correct as a short ball allows the approaching player to hit more angles and generate more pace due to her forward-motion. Nevertheless, too many times the short ball come in fast, with a lot of spin or a lot of angle thereby making it unsuitable to approach. I believe that there should be a shift in emphasis from location of the bounce to either (a) timing; or (b) opponent's positioning (vis-a-vis his baseline "T"). As a form of playing and practicing, the rule of thumb should be: (I) when you have an extra half-second to set up and crank it, come in; (II) when the opponent is 10 feet (or other arbitrary location) behind the baseline or outside of the side-lines - come in! Forget about where the ball bounces on your side and focus more on punishing your opponent for either (a) bad positioning on her part or (b) poor shot selection on her part (despite having hit the ball deep).
Initially, practice this by having balls fed very high and deep into your own court forcing you to move back. The player should back up, "load and explode" after that ball and immediately "bolt" inside of the baseline. Rinse and repeat a set of ten. Follow up by doing this drill where the player has to rush in after the "deep approach" and take the next ball out of the air. In live-ball training, the coach should place certain "markers" (e.g., strips of athletic tape) about 10-12 ft behind the baseline and 1-2 feet outside of the service line. When the player sees the rally-opponent (coach or another player) fall outside of these zones, the player should come in - regardless of where the ball is on the opponent's side. The goal should be for the player to incorporate an additional dimension for "the right time to approach". Come in (or seek to come in) when you have time to set up and belt your shot; come in (or seek to come in) when the opponent's out of position.