Here's what you do: Hold the corners of your mouth with your pinkie fingers and stretch the sides of your mouth (I could only show you a one-handed version here, because my other hand is taking the photo -- duh -- but the two-handed thing is important to make this stretching action work). Don't do it barbarically or anything -- I think it feels good, like a little self-massage. You can also, right before getting your picture taken, open your mouth as wide as possible (yawn-style) and then try to smile at the same time. If I cared, I would maybe turn around and cover my face while doing this so that that isn't the expression that gets captured on camera -- all up to you. If you want that puffy top lip look favored by some xojaners (hi, Cat and Julie!), you can stretch downward so that the top lip looks a little bigger than the bottom one.
Here's what it does: It widens your smile (if you like that sort of thing) and it relaxes your facial muscles so that you can smile more naturally -- or frown more naturally, or make whatever expression your face wants to make with less tension (less tension=gorgeousness, in my opinion).
Okay, that's more than enough on that little subject. xo