It's entirely possible that I harp on these more than most for two reasons. First, I have occasionally rushed in where angels fear to tread. Secondly, back when I was a volunteer rape prevention instructor, these were the two prime tools in the self-defense arsenal.
Of course, it feels odd to refer to mentality and situational awareness as two separate tools, even though they are. For me they are so intertwined and perpetually 'on' that they're one and the same. For me, you can't have one without the other, but I digress, on to the meat.
A lot of pundits have stated that mentality is the single most important element to a personal self-defense strategy and I have to agree with the caveat that situational awareness must be part of it. That's, partially, because predators can tell when someone is watching their entire environment and when that is combined with the proper mentality they will look for other targets. I scan everyone, everywhere I go, my wife says it's like I'm 'sizing them up for a coffin.' An accurate description and I've noticed that damn near every like-minded and similarly trained person I know does the exact same thing.
So, what is the proper mentality? I imagine most people will already know this at least partially, but few take it far enough. "Be polite and courteous to everyone, but have a plan to kill everyone you meet." Glib soundbite perhaps, but spot on. Another one is, "Watch hands, hands kill, in God we trust, everyone else keep your hands where I can see them." Another way that situational awareness ties into mentality. At any given moment, whether at work, home, or other safe environment, walking down a dark alley following a skip, or frequenting a bar in the wrong part of town, one should constantly be scanning your environment. Not jerking your head rapidly side-to-side with every step or stopping every few seconds to do a full circle, that just makes you look nervous. It takes a lot of practice to take in a full 360 every ten to fifteen seconds without being conspicous or running into things. Which is yet another reason to do it ALL THE TIME, practice makes perfect. So, you see someone, doesn't matter where or what he/she looks like, now you look for their hands. Where are they? Could they have a weapon? Are their hands positioned like they could retrieve a concealed weapon quickly? Now you look at posture and attitude. Are they slouched like they're not going anywhere soon? Are they poised like they're going to sping? Are they aware of their surroundings? Looking around? Or absorbed in something they're doing? Here's where it gets fun, are they projecting a false image? Trying to look relaxed and inattentive when they're actually ready to move and act on a moment's notice. Fun. The best part is you should learn to take all this in, in a roomful of people in just a half second or so per person. Hence, lots and lots of practice.
So, is that situational awareness in that you're being aware of your surroundings, or mentality because you're actively engaged in a defensive measure at all times. Both, but then you all knew that by now, didn't you? A defensive mentality must include alertness at all times, as outlined above, but also includes a great many other aspects, which I will try to cover in as much detail as possible. It's impossible to overstress the importance of these two things however. An otherwise completly unprepared, untrained, unarmed person still has a chance to protect him/herself if they recognize a threat early enough. For someone with a little training and firearm or other weapon, well, forewarned is forearmed.
Last thing, once you've mastered the threat scan (all the time, all the time), start playing wargames in your head. Yes, it's complicated and eats up your concentration a bit, but you should be playing the 'if/then' game in your head. 'If emo kid to the left pulls his hand out of his jacket and he has a knife in it, then I'm drawing down on him.' Again, it's all in practice. I'm not sure when it became subconscious for me, I've been doing it my entire adult life, but I'm only very vaguely aware I'm even doing it. My wife has gotten to the point she doesn't realize she's doing it unless I ask a specific question. She can respond instantly without thinking about it at all.