Gauge your opponent's height. I was playing doubles last night against 2 guys, one was like 5'3, and the other was around 6'3. When shorty came up, I'd lob it over him. He kept getting frustrated, trying to run back, and he kept whiffing on the ball. But his partner, I'd keep it low and fast over the net right at him, and he kept popping it up, where we would clobber it. He was too tall, and could get to a normal lob with his long arms, but he wasn't getting low, and had trouble with those waist high shots.