Write down your references. Write out or save a complete citation for each source you find; you may need it again later. Use citation management systems to help you.
Search multiple databases to be sure you don't miss the very best book, article, or summary on your topic.
Use the one good resource approach. When you find a good article/resource, look at the bibliography to see what resources that author used and try searching for them.
Ask a librarian for help!