Please note: As your librarian, I am more than happy to help you with choosing good keyword. If you're having trouble finding what you're looking for, send me an email using the contact box to the left (or come visit me in the library!).
Choosing good keywords is about 80% of good searching in databases, and will get you what you're looking for far faster. It's important to remember that everyone has different terms for the same concepts. For example, if I'm researching genetically modified organisms I could enter "GMO," "genetically modified organisms," "genetically modified foods," etc. and each would give me different results.
Before you start searching, think of your main topic, different terms for that concept, and some subtopics. I may search any of the following to find information on my topic:
"genetically modified foods and health"
"genetically modified food and safety"
"genetically modified organisms and farming"
As a librarian, I NEVER do one search and get exactly what I'm looking for. Do a few searches to see what gets you the best information for your topic.