If your opponent is one who analyzes an opponent's game and adjusts to it, and is able to identify patterns and take advantage of it, you need to be unpredictable and vary your tactics. But many players, probably most, do not do this. If your opponent is not one who adjusts, then don't make the mistake of "over-thinking," where you try to mix up tactics that are lost on your opponent. Against the non-adjuster, just use the best tactic in any given situation while not overusing any one serve, shot, or technique so much that he gets too used to it. It's against the smart opponent who can adjust that you must regularly vary your tactics.
Now go look in the mirror. Are you an adjuster or non-adjuster? If the latter, it's time to adjust your thinking!!!