Hire a good photographer for your photographs or simply enlist the help of a friend who understands how to photograph human subjects. More often than not, the trouble with photographs is not the subject but the photographer who needs to grasp the concepts of lighting, image composition, and human-to-camera interactions to capture authentic pictures of a person. Non-photogenic pictures often stem from the discomfort that many people feel toward a camera, which unnaturally comes in the way between the familiar face-to-face interaction of the human subject and the human photographer. A good photographer will understand this discomfort and work to capture a subject in a moment of comfort and authenticity.
Take care of your personal hygiene. Keeping well-groomed improves anyone's picture regardless of photographer, environment, or pose.
Pose in a complimentary way for the camera by maintaining good posture. Relax and hold up your head, keep your shoulders from hunching, and stand or sit straight. You may also discover that you prefer the appearance of one side of your body by observing both sides in a mirror; so when you pose for a picture keep in mind which side of you is facing the camera and angle yourself so that the photographer takes pictures of your best side.
Avoid harsh lighting or direct sunlight. These lighting situations may force you to squint.
Relax in front of the camera and behave as you would with a friend. Relaxing helps to bring out the natural beauty you possess off-camera and helps to circumvent the artificial environment of a photograph.