When you are photographing people one of the most important things you will want to capture is their expression. The last thing people want to see is something that looks fake or posed, so it’s your job to help guide them into showing something that looks real.
The first thing I recommend doing is to be ready to click the shutter the moment you make it known to your subject that you are going to photograph them. If you have your subject sitting in front of you, ready to be photographed, and you tell them to smile then they will do just that. After asking them to smile (and this is what not to do, by the way) you move into position and realize you need to change your aperture/ISO/focus point … meanwhile your subject’s smile is slowly fading and turns into the “open-mouth-teeth-clenched” smile. Unless they are posing for their dentist then it’s probably not the expression you were looking for. So my advice is to ask them to relax, take your time to get everything ready with your camera, and then bring your camera to the ready and then ask them to smile/laugh/etc. Doing it this way will make it much more likely to get an interesting and realistic shot.
Another thing that is important, but not always easy to do, is to make a fool of yourself. If you want to capture a smile then don’t ask them to smile, ask them to laugh (or somehow figure out a way to make them laugh). Tell them a silly joke, make fun of yourself … do whatever it takes to get them to laugh at you. In a photograph a laugh is much more likely to look like a genuine smile. This works extremely well with children who tend to give the cheesy fake grin when they know they are being photographed. Another benefit of doing this is that it relaxes your subject. For some people it’s a big deal to have their picture taken and they might be a little bit nervous, so making the experience playful and fun will help lighten the mood.
My last piece of advice is to figure out good descriptive words that people can understand and interpret into expressions. We don’t always want our photos to be happy and smiley; in the photograph above I was going for a more serious expression. I asked the model to “look tough”, and that was her “tough” look. It’s also important to understand that everyone interprets emotion and expression differently, so come up with several words and find out which one works best with your model to get the look you are going for.