Choosing your search terms and combining them into a logical query is a critical step. If you do this poorly, you'll get poor results. Databases are only as good as the questions you ask them! While search engines like Google do best with strings of words, other more structured databases can offer you more refinement.
The first thing to do is separate your question into its component concepts. Say you're looking for a procedure to determine Anatoxin-a in water using chromatographic methods. That's three distinct concepts: