If it bends, bend it!
You've seen the snapshots, sometimes of entire families, where everyone looks kind of like they're standing at attention--straight arms, straight legs, straight-on to the camera, no head tilt. This may be good for soldiers, but it makes for a pretty boring individual or family photo.
Look at this photo of Lydia, one of the high school seniors that I had the privilege of photographing last spring. Her knees, elbows, wrists, even fingers are attractively bent, and her head has a pleasing tilt to it.
Just think how different this photo would have come out if Lydia had been sitting straight on to the camera, hands folded in her lap!
So how do you get people to bend all those bending parts? By making suggestions...like putting a foot on a rock (which of course, automatically bends the knee), or putting a hand on a hip or in a jean pocket, or crossing arms in front and leaning against the person beside, or maybe, tilting heads toward each other (which gives the feeling of relationship rather than just a bunch of people looking at a camera).
So remember, to make pictures all that they can be, encourage people to bend whatever bends!