One of the most important factors in searching is to document your search. You want your study to be reproducible.
You also want to be able to refine your search, so it is essential you detail it.
It is a good idea to use a Word document (or similar) to keep track of your search.
When searching, consider keywords, synonyms, types of studies, and spelling variants (i.e. color vs. colour, randomised vs. randomized).
In order to refine your search terms, it is always good to refer to relevant resources to find new keywords and search terms. You can also use citation searches to find additional authors and studies.
When searching PubMed, you may find the Yale MeSH Analyzer helpful.
Make sure that grey literature, hand searching, and nontraditional searches are all a part of your review.
You also want to limit bias by searching for international studies (including in other languages).
You can also ask your librarian for assistance.