Sometimes you gotta uglify something before you can pretty it up. I felt a little obnoxious about putting a nasty, tsk-tsk sign in our shared bathroom. But it was only there for a week.
Nagging is ugly, too, and besides feeling petty, it's proved singularly ineffective. One problem is that nagging happens long after the fact, not at the moment of toothpaste yuck-ification. But a sign right at the place of the dreadful act? Would "pre-nagging" work?