Let's assume that you are a conscientious driver who actually uses turn signals regularly. That's a good start. However, using your turn signals is only effective if you use them at the right time. For instance, if you pull up to a red light with the intention of turning left and wait until the light turns green before using your turn signal you are a total jackass. Why bother? You might as well just honk your horn instead. It's too late for the guy behind you to pull into the right lane and go around. The only thing worse than that would be waiting to turn without using you signal at all.
Another example of turn signal jackass behavior is turning your signal on prematurely. If you are heading toward an intersection and intend to drive straight through and then turn at the next cross street, don't turn your signal on until you are past the first intersection. Otherwise people are going to think you're turning at the first intersection.
I had the good fortune of learning how to use turn signals correctly, and one of the fundamental principles of good turn signal usage is turning your signal on BEFORE you apply the brakes. Don't slam on the brakes and then put you blinker on after you've begun to round the corner. That makes you a jerk. Incidentally, the only exception to this guideline would be braking through one intersection to slow down and turn at the next cross street. This is a bit of gray area. Simply activate the appropriate turn signal as soon as you have cleared the first intersection, even if you must apply the brakes beforehand.