When I see people with clean, big, unused trucks, I always give them this hand signal:

Wag your index finger, and it looks like their small limp dick, blowing in the wind. You get double points if you look them in the eyes. They usually get the message. Be careful, though. I’ve gotten into a few fights over just that hand sign.

I’d definitely reserve it for those pissers who ride your ass or park like assholes, but get ready for a fight.