If you need to use any "unfamiliar" toilet (public or just at a someone's house) make a habit of grabbing a couple sheets of toilet paper first and wiping down the seat. That way you'll never sit on a wet seat and you'll know in advance that there is toilet paper.
If you're having a yard sale or just want to make a sign, an easy method is to use 3-sided tomato cages that fold flat. Unfold the cage and put your sign inside. Tape it in place across some of the wires to keep it from sliding up-and-down then fold the cage sides back onto the surface to keep it locked in place.