Funny you should mention this...
Last night, in our Mage game, we cleared out an old node that was being inhabited by evil fae, a vampire, and a big spider monster. Not wanting other bad guys to come along and take up residence, I was thinking about scrawling a warning on the walls:
"This area under the protection of the Three Amigos. Trespassers will be executed."
We had all dressed up as a mariachi band as part of our disguise.
In addition, I was thinking about carving some kind of complicated glyph on the floor--something unique that we could scry on and teleport to in the future.
A long time ago, Pielorinho ran a game where he used wall-markings to great effect. There were these very faint blue dots hidden along the walls of certain passages. Thinking that we were very clever for noticing them, we followed them...straight into a very nasty trap!
Spider