Rule 2 states: Be aware of your surroundings. Situational awareness is paramount.
ITS Tactical has a good article with a simple explanation on how to start training your situational awareness and why you should do it.
Developing awareness is a skill. At first it will seem very awkward and self-conscious, but with practice, it will become seamless and subconscious. You will start to pick up on more and more subtle rings of disturbance and more complex stimuli. Eventually, people may think you are psychic as they notice how you seem to sense events before they unfold.