Keep your subject in familiar surroundings. Get him or her to relax, to forget the camera. Assure your subject that he or she looks good. Patience and good nature will take you a long way. Try several different poses to emphasize attractive features.
Children are another story.
Unpredictable, impatient, and often reluctant, they'll test your skills. Do not try to impose your will. Keep the session relaxed. Give them something to do and let them move around. When the child tires or becomes disinterested, quit.