TOO MANY RESULTS
Check subject words in the best article(s) and use them in a new search or try subject searching with a thesaurus
Add a new concept that is part of the topic. Use the "AND".
Search terms in an index (title, subject, or abstract).
Ensure phrases are searched together. Use quotations or brackets.
Force terms to be within a certain distance (bicycle 2/Near helmet) to get more relevant content. Proximity searching
Try a discipline specific database.
Try limiters (publication date, peer reviewed)
TOO FEW RESULTS
String synonyms together with an "OR"(Math OR Mathematics OR Calculation).
Truncate to get many variations of a word(diabet* for diabetic, diabetes etc).
Try a multidiscliplinary database like Search it all.
Is the topic academic or is information more likely to be found in news or government publications.
Broaden your search from a keyword in title to a keyword in abstract, for example.