Using quotation marks in your query ensures your results contains words and terms you search. Instead of searching for the words individually (think "Diet" "Coke") it will search for them together ("Diet Coke")
Using an all-caps AND, OR, NOT, you can link terms.
AND is a great way to pair two different search terms (think "United States of America" AND "Unemployment rate").
OR searches for both terms ("United States of America" OR "U.S.A.").
NOT excludes terms ("United States of America" AND "Unemployment rate" NOT historical).
Check out the filters on the left side of the search tab. Especially make use of date and subject.