The next time your opponent is having a hot streak, simply ask as you switch courts, "What are you doing so differently that's making your forehand so good today?" If they take the bait--and 95 percent will--and begin to think about how they're swinging, telling you how they're really meeting the ball out in front, keeping the wrist firm, and following through better, the streak invariably will end. They will lose their timing and fluidity as they try to repeat what they have just told you they were doing so well.

The greatest lapses in concentration come when we allow our minds to project what is about to happen or to dwell on what has already happened.

When we plant a rose seed in the earth, we notice that it is small, but we do not criticize it as "rootless and stemless." We treat it as a seed, giving it the water and nourishment required of a seed. When it first shoots up out of the earth, we don't condemn it as immature and underdeveloped; nor do we criticize the buds for not being open when they appear. We stand in wonder at the process taking place and give the plant the care it needs at each stage of its development. The rose is a rose from the time it is a seed to the time it dies. Within it, at all times, it contains its whole potential. It seems to be constantly in the process of change; yet at each state, at each moment, it is perfectly all right as it is.

Lots of good tips about improving mental performance and concentration. Not limited to tennis, is applicable to any activity.