Many players have multiple options for most situations, but only one for some. For example, against a deep, spinny serve to the backhand, many players will only backhand drive it crosscourt. Or against a backspin serve to the backhand will almost always push it crosscourt. (Crosscourtitis is a curse many players have – there is such a thing as down the line, and you should learn to use it.)
When you have only one option off something the opponent does, then the opponent no longer has to worry about anything but that one option. And since a player with only one option usually only has that one option because he's not particularly comfortable with the incoming shot, it usually means the one option he uses isn't very strong. But even if it is, it loses its effectiveness when the opponent knows it's coming.
Even if the opponent isn't a "thinking" opponent, i.e. one who figures out opponent's weaknesses (such as predictability), most players are instinctive, and subconsciously pick up on these things. They may not realize it at the time, but they often are reacting to this predictableness.
So examine your game, and find places where you generally do the same thing over and over. It's possible that this works against some players, or even most players your level – but it probably doesn't work against stronger players, and presumably they are the ones you are hoping to learn to beat. So make sure that in every situation, you have at least two options.
Here's an example. During my playing career I often liked to give big breaking sidespin serves deep to the backhand, so that the ball would break to my right, away from the righty's backhand. Most would reach for the ball and make moderately aggressive shots to my backhand – but I'd already be over there, just waiting for this shot with my forehand. The ones that gave me trouble would either take it down the line – often doing so a bit more quickly and catching me – or would simply mix in a chop now and then, which would completely throw me off, since I was generally a step back, waiting for that topspin return.