I think it is extremely important to stay with what is working in tennis.
Many players get so wrapped up with fooling their opponent and proceed to change their ideas on almost every shot and point. All that is necessary to keep your opponent off balance is to do the unexpected once in a while.
For instance, if you usually hit your groundstrokes crosscourt, play a point by hitting primarily down the line. If you usually stay back at the baseline, come in to the net on a point. If you generally play consistent and steady, add an aggressive point here and there. The important part of the variety idea is to go back to your strength and use what you have been successful with most of the time.
It is usually better to hit your shots well, even if they are a little predictable, rather than opting for variety and not executing the shot properly. The best players in the world are not worried about their opponent's ability to figure out what they are going to do. They still feel like their best shots will be tough for their opponent to handle, even if they know what is coming. This is the same mentality we should have.
Try to get the majority of the points played to be on your own terms, and throw in some variety once in a while.