Develop your search strategy - use PICO concepts to craft search statements.
Create one search statement to represent each concept; be specific; consider synonyms or more specific or broader terms.
Use Boolean operators to combine search statement in various ways.
Apply appropriate limits (dates, language, publication types, etc.)
Save the best citations from each set of results;
Refine the search strategy as you go.
Search is an iterative process. Continue searching as long as you still find new literature.