When someone comes up to the shop that I don't recognize (and they're not driving a Jeep), I get my son's attention and we both focus on the person until we determine they're not a threat.
I also built a chain fence to go across the bay door so nobody can run into the shop and catch us by surprise.. again.
Whenever one of us walks outside, the other stands watch until we determine the parking lot area is clear.
It might sound like paranoia, but it has become automatic for us and doesn't really hamper production. The robbery taught me that "it can happen to anyone.. anytime." It sucked looking down the barrel of a gun but it was a good learning lesson..