Once you've been able to narrow down a location and time period for an old photograph, your knowledge of your ancestors comes into play. Where did the photo come from? Knowing which branch of the family the photo was passed down from can narrow down your search. If the photograph is a family portrait or group shot, try to identify other people in the photo. Look for other photos from the same family line which include recognizable details - the same house, car, furniture, or jewelry. Talk to your family members to see if they recognize any of the faces or features of the photograph.
If you still aren't able to identify the subjects of your photo, create a list of the ancestors which meet all of the possible criteria, including approximate age, family line and location. Then cross off any people who you have been able to identify in other photos as different individuals. You may find you only have one or two possibilities left!