Trust your instincts! In order to do this you have to, first, recognize that you DO have instincts. Once you've made this realization, you will be able to practice moving with a heightened sense of, 'situational awareness'. Recognize, 'When' you need to be more alert. Here are some of the usual, 'threat alerts': Anytime that you are in transit; anytime a stranger is attempting to get close to you; anytime someone, else, is paying too much attention to, either, you or your movements; and anytime you recognize that you are in a, 'high risk' environment.
Here's some of the more subtle stuff: Anytime it, just, doesn't, 'feel' right; anytime you can't see the hands; anytime the other guy is too alert; anytime you sense tension in another person especially when that person suddenly, 'relaxes' as he gets close to you; anytime you notice another person looking around himself; and, last but not least, be EXTREMELY careful with women - they can be very dangerous!
Good habits I've developed over the years: Use windows and mirrors to look around and behind yourself. Stop occasionally and, 'check your back-trail'. This is a tough one: In crowds, stop suddenly, put your back to the wall and look around yourself; it takes a lot of experience to learn how to filter through so much visual information; consequently, teach yourself to look for faces and eye contact - first. When you walk through a doorway, step to either side as you enter and quietly glance around. Become aware of, 'corners'. They can be very dangerous. Sweep the area BEFORE you pull in to park; and remove your seatbelt well-before you stop the vehicle. Always carry, at least, two weapons; and keep one in a front outside pocket.
In general, don't make it easy for others to, 'track' you: Get caller ID and an answering machine; let the machine handle the, 'Unknown Caller' numbers. If possible, vary your travel routes and times. If you can't do that, then, be aware that this is, 'high risk' time for you and act accordingly. Learn to appreciate the high value of, 'backup'. (The Lone Ranger never would have made it without Tonto!) Finally, EXPECT to make mistakes because you will; and, sometimes, the other guy(s) will have the advantage. You need to be able to recoup from these situations, too. In the final analysis, you have to be a little bit smarter, a little bit quicker, better balanced, and a little bit more ruthless than whomever you are up against. Learn to deal in the unexpected; recognize that, ‘surprises’ are the norm!