We all like to think everyone we live with is trustworthy. Unfortunately, this isn't always true. To protect yourself and your belongings, keep your door locked, even if you just go down the hall for a few minutes. Engrave expensive belongings with your name, or an driver's license number. This will help police recover your items if they do get stolen.
Propped doors allow anyone access to the hall at night, and jeopardize everyone's safety.
Walk with someone you know, whenever possible. When you have a friend come for a visit, let other people on your floor know about it. That way they won't think there's some strange person wandering around the hall. Keep these phone numbers posted near the telephone. This list could include your family, friends from home, friends at Cal Poly, medical information, etc.