Having an appropriate topic scope is critical to successfully finding and appraising literature. If your scope is too broad you're search will be full of irrelevant hits, but too narrow means you may find nothing - not even similar articles. Before you search you should define your research topic and the scope of your project. Several strategies to do so can be found below.
Guide from the University of Melbourne Library on the different stages in the research cycle. The pages entitled 'Plan' and 'Identify' have some incredibly helpful tips on what to do before beginning a your search.
Guide from the Weber State University Stewart Library on the steps to take when selecting a research topic.
Nice video (3:42) from the Pfau Library at California State University, San Bernardino on how to choose a research topic that is of a reasonable scope and size.
There are several different mnemonics to help you map your search. Some will be more applicable than others depending on the discipline and subfield your topic fits into.
PICO: Patient/Problem; Intervention; Comparison; Outcome
PPAARE: Problem; Patient; Action; Alternative; Results, Evidence
Concept Breakout: start by breaking your topic into its major concepts. This is similar to what you do when you have a clinical question but you are not confined to thinking about your concepts as either a patient or an intervention or a comparison or an outcome. By doing this you will be able to match concepts to keywords and controlled vocabulary terms and break your topic into discrete, searchable pieces.