The overhead; what's the overhead?
You're up reasonably close to the net; you're bothering your opponent. He hits a lob because you're bothering him, he wants you to move back. A great drill for us to practice going back is to stand with the racket touching the net. Now, you're not doing this in a game, this is just for exercise purposes and a challenging drill. Touching the net, the pro hits a lob over your head, the first thing you do is you step back, move back, arm up, look up, strike the ball, finish the swing, admire your shot. No, come back to the net and touch that net again. It's exercise as well.
So, the drill is run up to the net, run back, hit the overhead, run up to the net, run back and hit the overhead. We do it over and over and over again, until your body does it without even thinking. The overhead motion is very much like the serve. It is the serve, essentially, but it's the serve at the net. As Kirk mentioned, you're going to touch the net on this drill, you're going to get ready and you're going to run back in the overhead position, or what we call the scratch back position.
You're going to run back a couple of steps, reach up, hit the ball like a serve, follow through, and come back up to the net and touch the net again. That is probably the most well-used overhead drill in tennis. Touch the net and go back.