Park in well-lighted areas. Check the area before leaving your car at night. Look around and under your car when approaching. Check the back seat of your vehicle before entering. Remove valuables from unattended vehicles or lock them in the trunk.
Every situation is different.
Only you can be the judge of what action is appropriate. Be realistic about your ability to defend yourself. Resistance or yelling may help, but it may also lead to further harm. If your life is in danger, passive resistance may be your best defense. Should you be victimized, notify 911 and Campus Security immediately.