I always thought "You are my crumple zone", "my brakes are 30 years old" type stickers were a good way to suggest people should maintain a good braking distance (and not cut me up!).
I don't need a bumper sticker to warn people off, just the look of my old landy is enough to make them keep clear. Although, having said that, I had to stop on a hill recently, in a line of traffic, quite a steep hill, and the plonker behind got as close as possible. I let Compo roll back a couple of inches before I pulled away, the plonker kept a better distance after that.