Try different keywords and search terms using different databases and catalogs. Every database is different so some keywords and search terms work for one database but not for another.
Expand your search by:
including synonyms and plural/singular forms of keywords. Separate synonyms by OR. Separate the synonyms from the rest of the words by using parentheses.
use truncation symbols (or wildcard symbols) to search variations of your search terms (e.g. scien$ for sciences, scientific, scientfically, etc.) See truncation guide for truncation symbols.
Narrow your search by:
combining different concepts/search terms with AND
using the limit functions of the database. Look in the database's HELP function to discover the limit functions it offers. Possibilities include limiting by date, language, type of article, etc.
Keep a record of what search terms work and which databases work for your topic. This can help you from repeating your steps.
Did you find an article you really like? Then, read the cited references (a.k.a. bibliography, end notes, footnotes) to find similar articles. Unfortunately, this can bias your project by focusing on only one side of an issue.
Ask for help. Ask a librarian for search tips. Also, use the help screens in the databases for instructions and tips.