What is the best thing to do to other drivers who piss you off in traffic?

So far being in front of them and keeping your windshield washer pumping to soak them is about the only thing I can think of that you can get away with.

Of course, dumping chicken feathers is legal, but everything else I can think of will land me in jail.

Anyone know of anything that is really effective, but not going to cause a traffic crash and pile up?