After dark always walk with someone. Statistics prove that attackers usually prey on lone, unsuspecting targets. An attacker is less likely to approach a group and will sit and wait for a lone target to arrive. Remember, the larger the group, the less chance of being attacked.
Always let someone know where you are going. Also, keeping a noise making device (whistle, canned air horn, etc) with you when walking at night is also a good idea to attract attention as well as to scare any would-be attacker away.
If you must work alone in a campus building after dark, be sure a friend knows where you are and when you'll be home.