Monster, you asked about tennis-related exercises a few weeks back - well, I have one for you now. The exercise is some sort of modified lunge, and while I can't really say how it's different from a regular lunge, I can tell you after having done it a few times that it hits some different muscles than I normally hit with lunges, namely it hits the upper hamstring and butt area harder than I'm used to. I came about it because I have been bending from the waist to hit low volleys, but the proper technique is to keep your back straight as much as possible - ie, bending down at the knees, which more or less results in a lunge. When you have time, there's a crossover step to do for the volley - so for example on a backhand for a rightie, your right leg will cross in front of the left one, I'd say at a 33 degree angle (twice as far sideways as forward). And if it's a low volley, the next thing to do is bend at your right knee and lower your body toward the ground, so that your upper body's stance remains as it would for a higher volley. This is more or less a lunge, but modified for tennis. Maybe someone can find a good pic of a pro doing it, I'm sure they exist, but I'm too lazy to go searching
Anyway, give it a try. To be honest, I'm not sure how well I can do at it - at 39, getting down quickly isn't so easy. But I'm definitely going to do this motion (in fact, shadowing the entire stroke, to bring up another practice technique you mentioned to DSter recently) instead of (or maybe in addition to) my previous lunge routine.