Aside from using AND, OR and/or NOT as Boolean Operators, here are some other tips:
PHRASES: Place phrases in quotes....example: Instead of Acute AND Chest AND Syndrome try "Acute Chest Syndrome"
MORE ADVANCED: You can try Truncation and Wildcard Searching:
Wildcard & Truncation Searching
Also, look at the ENTIRE PAGE of your search. Are there more limiters on the sides that will help direct your searches? Full-Text...Peer-Reviewed....type of study....etc.
Examine what other terms appear and create new search terms.