When a driver squeezes me towards the curb or acts like I am not existent in the traffic, I tap the side of their car with my hand when they're very close. Then they think they hit me or my bike and suddenly become very polite and say they're sorry.

It is my thought that if you try to warn them without any kind of incident (without the tap), than they may become rude. They usually think, well I didn't hit you, did I, so what are you bothering me for.