TOO FEW hits: OR to add synonyms; individual key words, NOT whole phrases; drop concept(s) with AND; truncate; consider other databases.
TOO MANY hits: AND another concept into search; use other available limiters in the database.
Truncation symbols (?, *, !, +) will provide variant spelling after the root word.
A wildcard (?, *) is a character that may be used in a search term to represent one or more other characters.
Do an advanced search and take advantage of the fields provided.
Use the subject headings/descriptors to find additional citations on your topic. If that does not work, use the keyword search.
You can also use limits (e.g., publication year, language, words in the title, etc.) to narrow your retrieval.
Set up an auto alert/RSS feed to monitor the research.