Set a date to periodically review those families you have not worked on in a while. It can be easy to get wrong ideas, misconceptions, or incorrect memories about your families in your head when information has not been viewed in some time. New databases or websites might have become available since you last looked at the family or you might have learned more about research in general since you last worked on the family.
Maybe your ancestor's birthday, anniversary, etc. would make a good day to annually review your files--especially on those people you gave up on or thought you had "completed."