1) NEVER let your guard down in public. EVER.
2) Be aware of your surroundings.
3) Scan for threats.
4) Check your 6.
5) Learn martial arts.

I'm not saying that would have prevented the crime, but I'm guessing it would have helped. I've been sucker punched. I walked away (luckily) because there were 4 others with him. Since then I've earned my black belt and I would still try to get away first, but if I couldn't then the lead guy is going down first. Sometimes it's enough to stop the gang mentality when you drop the instigator of the attack.

Don't know if a gun would have helped here in the least.