Here’s what I say to folks when I’m trying to get them to look natural in a picture. “Act like you know each other!” They ususually laugh and smile, naturally, and move in closer to each other.
If it’s a single person, I will often tell them to gaze off into another direction, rather than to stare directly at the camera. It looks more natural, or I will tell them to face the camera, but have their eyes looking over to the side, then I’ll say, “Think of a happy thought.” That usually gets people to smile a natural smile (even if it’s just a closed mouth grin) but it looks better than a big fake smile. If someone is just standing and I want their arms to appear natural, it’s best to give them something to hold, like a flower or a book or a hat or a spatula (if they’re in the kitchen).