Get Closer
The most common mistake made by photographers is that they are not physically close enough to their subjects. In some cases this means that the center of interest—the subject—is just a speck, too small to have any impact. Even when it is big enough to be decipherable, it usually carries little meaning. Viewers can sense when a subject is small because it was supposed to be and when it's small because the photographer was too shy to get close.
Don't be shy. If you approach people in the right way, they'll usually be happy to have their picture made. It's up to you to break the ice and get them to cooperate. Joke around with them. Tell them why you want to make the picture. Practice with people you know so that you are comfortable; people can sense when you aren't.
The Casual Portrait
Wherever you are with your camera, always be on the lookout for those moments when a person's character shines though. If you have a formal portrait session with someone, make some frames of him while he straightens his tie or while she brushes her hair before the formal sitting. Walk back to the car with her and shoot her on the street. If you are on a spring picnic with the family, look for that moment of bliss when your wife leans back, sated, to enjoy the caress of the warm sun. If you're on the street, look for the impatient expression on a pedestrian's face as he waits for the light to change. Always be on the lookout for the telling moment. Every person has a story, and every picture should tell part of that story.