We had a bit of theft in the dorm I lived in at Purdue. The interesting thing was the residents figured out who it was and took care of it themselves. No, we didn't lay a finger on the guy, but we didn't have any more problems.
1. Don't leave your stuff laying around. The library, a study room, a class room, bicycle racks, and especially the laundry room - I'll be right back is long enough to find some things missing.
2. Text book theft is a big deal. Those books are worth hundreds, there is a ready second hand market, and they are pretty much untraceable. Easy cash. Second on that list is calculators.
3. Live in a dorm? Check your parents' homeowner insurance. You may be covered. Live in an apartment? Get renter's insurance. PERIOD.
4. If it can be locked, lock it. Start with your dorm room door.
5. Bike locks? The heavier, the better. Cable locks aren't worth a dime. I had one cut. If you have an expensive bike, keep it in your room.
6. Develop friendships in your dorm. When people know you, they look out for you. I remember getting a phone call from a guy just because he hadn't seen me at dinner for a few days. (I had to change my schedule.) Just checking up on me. Today's "modern dorms" with private baths and everyone having a fridge, TV, computer, game system, etc. in the room keeps people out of the common areas and dilutes the "college experience" in my book.