Stay away from prints if possible. Never wear loud prints - they are very distracting. For large groups, ask if the camera can be repositioned before making your appointment. If your group has more than 4 subjects, it is important to be able to change the camera angle to accommodate everyone. If your studio has a digital monitor reject the photos that are not perfect.
Many studios can store the photos to go back over after the session.
Don't settle for 'okay' portraits. Keep in mind that the camera picks up more than the monitor actually shows. When your photos arrive feel free to reject any pictures that are off center. If the subject is not centered properly, the photo will not frame correctly. Also reject photos where the color is off, someone blinks, the edges of the background are visible, lighting fixtures are exposed, or are otherwise imperfect.