A more recent one that I've always thought was useful is: If you can't see my mirrors, I can't see you. It's a good reminder, a simple way to stay visible to trucks and other vehicles on the road.
I suppose these stickers have become a bit too common. Or maybe there is just a trucker out there somewhere who is trying to jolt me into awareness. It is, after all, very dangerous to pass a truck on its right, especially when it is in the process of making one of those wide turns they love so much.
At any rate, today I saw a big truck in front of me with two stickers on the back. They were both standard bumper-sticker-size, both in simple black and white. The one on the left read: Passing Side, with an arrow pointing left. The one on the right, with an arrow pointing right, read: Suicide. They surprised me until I voiced the words inside my head.....
It's certainly harsh, but it did a good job of shocking its point home.
Oh, and why do they call one of these things a "semi-truck" when it's so much bigger than other trucks?