I think 95% of situational awareness is achieved by living in the immediate present. This is done by not reviewing what was said at the office earlier in the day, nor dwelling on whether the grocery has sold out those sale items already while you are making your way through life and traffic. If you are just looking at, and often enjoying, your current environment you will note those things that are out of the ordinary.
You don't have to be paranoid, nor on "red alert", to look at each vehicle at the gas station when you pull up. The same for taking a glance at the vehicles that pull in when you are fueling, or at those that exit the store while you are there.
The only trick is to attune yourself to note the person(s) in the environment around you that is/are "different" and the courage to risk looking foolish to avoid them.
An instance of the above would be leaving after getting three gallons of gas, when you meant to fill up, because a the three passengers in a newly arrived car were making far too much eye contact.