I just built a new crosscut sled for my table saw. This must be number 10 or higher - I use them all the time. Sometimes my kids (late teens, early twenties) use them as well, and I'm kind of fanatical about their safety (and my own).
So here is my safety label, using red paint:
It might be a little graphic, but it has caught everyone's attention immediately when they set the sled on the table.
I may also put a wooden blade guard on the back of it, but I want them to be thinking about safety regarding the blade when it's doing the cutting in the body of the sled as well.
I might write DANGER on it...