First, I'd suggest that much of the advice already given about photographing children applies to people of any age:

• show them busily engaged in something that they like doing, and that is really characteristic of them.
• include some pictures of them just smiling at the camera if this is what you want! Instead of just telling them to smile, though, do something that will make them smile more naturally. If your subject is looking self-conscious, ask him/her to look away for a moment before looking back at the camera. This way you may get a natural smile.
• don't make people squint into the sun. Either experiment with back or side lighting (if necessary using flash as a fill-in), or choose a bright cloudy day, or, failing that, position them in the shade. Try to avoid harsh mid-day lighting. With older people, aim for soft, even lighting which may (partly) conceal their wrinkles!
• choose a camera position that lets you see their faces really clearly.
• don't use a wide angle lens, unless you really want to give your subjects extra large noses and tiny ears!
• avoid distracting backgrounds - and particularly any objects that look as though they are growing out of people's heads. If possible, arrange for the background to be out of focus. One way of doing this is to use the telephoto end of the zoom which reduces the depth of field. It's also more flattering to the subject.
• Arrange for your subject to lean towards you. This often produces a more lively result. It's also usually more flattering to look up at people from slightly below, as shown lower on this page.
• Have everything ready so that you take the photo(s) as quickly as possible.