Listed below are several preventative measures one can take to possibly deter unwanted attention, or discourage ongoing stalking events.
Increase your awareness:
Do not overlook the signs of unwanted attention.
Heed internal "red flags" (i.e., Intuition) alerting you to danger.
Harassment/stalking often begin as minor annoying encounters, be attentive to early warning signs prior to the escalation of the stalking.
Although "Letting someone down easy" may seem conscientious, it can inadvertently provide a "mixed message". Give a firm and definite "No", to communicate that you are not interested in a relationship of any kind.
When receiving unwanted attention, you shouldn't respond at all. If it is too late or unavoidable, don't use statements that can be misconstrued.
Any suspicious activities should be reported to the police.
Threats (from minor or vague to severe or specific) should be treated seriously and immediately reported to the law enforcement.
Protect your privacy:
Identifying or personal information should not be entered into online profiles/directories.
Encourage others to keep your personal information confidential.
Never give out your Social Security number over the phone or enter it online.
Use a private post office box
Target hardening / Personal protection:
Lock all your doors and windows.
Install motion detectors and/or a home security system.
Avoid walking/exercising alone.
Having a mobile phone and/or beeper can be a great investment.
Be more vigilant with your children and keep your pets inside at night and when you're away.
(Many of the Preventions are modified from The Antistalking Web Site and the The Privacy Rights Clearinghouse)