Some of the rules which I've heard/liked/follow:

- ask ahead. people will gladly tell you how scary a place is if you ask
- camp with other people. You're less likely to be bothered if you're not alone.
- tell someone where you are. just in case the worse happens you'll be found
- if you're going to carry a weapon, make sure you're trained AND be prepared to use it to it's full capacity. That means if you're carrying a gun you'd better be prepared to kill someone. Also know the ramifications of using it (I can't imagine letting pepper spray loose inside a closed van. No-one would make it out!
- always have an escape plan.
- be smart. wandering off alone or spacing out is an invitation to trouble

Personally, I've slept (sometimes camping, othertimes just sleeping) in all sorts of places I probably didn't belong. The times that I've been bothered the signs were obvious to not sleep there, but either I didn't see them or didn't care.

john