I have a sort of natural look to me that can best be described as a serious intimidating death-stare. Because I acknowledge this, I've learned how to use it for the better. This look is also euphemistically named "The F-off face", particularly when I'm trying to scare away mobs of people trying to get me to sign random petitions (or something of the such). It's more polite that way. I mean, if they're too afraid to talk to me, it saves us both time since I'm not standing there pretending to be interested. Then there's a more pleasant game I play where I walk around with that same unfriendly front, but if I happen to make eye contact with someone, I'll try to surprise them with my cheeriest "good morning!" with a big goofy smile on my face.
As I do in general, I tend to hold a grumpy look in photos. Though every now and then, I could stand for a smile.