Minecraft wiki entry for Traps, associated text: "Free Diamonds!" This could be good on a trapped chest, but would only work on newbies.
"DO NOT ENTER!" Most players would be tempted to do exactly the opposite, even into a possible trap.
"DO NOT PRESS!" If you see a button, you really want to press it. Especially if there is a sign saying not to.
"FREE ADMIN!" Like the "free diamonds" example above, this probably works on newbies only.