Photo Tip #30-Sex or Wednesday, but not Sex on Wednesday
Forget saying "cheese." If you really want people to smile, ask them to say "Wednesday." If you really want them to smile and they simply won't, ask them to say, "Sex." Almost no adult can say "Sex" without a slight giggle, though you might wind up with a smirk in post processing (and the magic word might work just once, so be quick on the shutter.)
To get a more natural look, I often ask people to look down at their feet, and remain looking down until I tell them to look up. Then, as I say, "look up" as they start to move their head up and their eye level reaches camera level, I expose the image to get a more natural, un-posed look.
Don't forget that you can sometimes create an interesting portrait by having somebody not look directly at the camera. Somebody gazing into the distance, either left or right of the frame, can create tension and interest in an image as well-it makes us wonder what they were looking at when you took the picture.