Start with one or two keywords, then combine concepts one at a time. Do not add limiters (date, article type, etc.) until you're sure your keywords are productive.
Use controlled vocabulary to refine and focus results. Many articles have standardized descriptive terms assigned to them so you can easily find related articles.
Did you find one really good article, but you're having trouble finding a second one?
Locate the article description in CINAHL, then click Find Similar Results (it's on the left).
Other PubMed tips:
MeSH database via PubMed: MeSH tutorial
Other ways to find the full-text of an article: