A highly-recommended 10-minute video from the librarians at Yale University.
Did You Find Everything?
Check the references of relevant articles: It is always helpful to scan the cited references of at least a few of the papers that are particularly relevant to your search. Do the author(s) cite any sources that your search missed? If so, what are some of the controlled vocabulary/keywords for these articles? Can you add these terms to your own search?
Do a cited references search: Select a few articles from your search to see who else cites them. This will help ensure that you have the most current information as possible. You can do a cited reference search in Google Scholar or Web of Science.