From a column by Anne Burke in the July 2012 issue of Via magazine: “In direct sunlight, use forced-flash mode to eliminate facial shadows”.
Have the camera ready before you ask people to pose. If your (digital) camera has an action photo option (an option to take photos so long as the button is pressed), use that option to take a series of photos of your subject.
Give your subjects a focal point. If your subjects (e.g. babies and cats) don’t understand verbal instructions, then provide an attention-getting object or sound for the focal point. Note that if your camera is the focal point, then a mobile subject may step forward to investigate the camera; this is the cause of many a closeup of a feline nose.