Creating a working list of keywords is critical for beginning your exploration of the literature on a given topic. When you search, you have to try to anticipate which words would have been used by the author of the article you are hoping to find. Since you're not psychic, your best bet is to come up with a list of possible keywords and their synonyms.
Begin by brainstorming your topic and creating a list of possible words to search. Let's use the example of anticipating the environmental impact of developing swampland for commercial use by a shopping center.
What words come to mind that describe or define your topic? (examples: swamp, mall, "environmental impact")
Is it part of a larger topic? (examples: sustainable, "economic development", deforestation etc.)
Are there narrower topics within your topic that you need to search separately? (examples: "parking lots", runoff)
Are there related words or concepts that should also be investigated? (examples: "carbon offsets", "sustainable landscaping)
Are there specific examples (proper nouns) that you can think of that you can search for? (example: DestiNY USA)
When you're done brainstorming, it's time to do some test searches, but hang on to the list as you begin your research.
Once you find a source, check to see if it has Subject Terms attached to it. Note any that seem useful that aren't already on your list. If they are hyperlinks, you can often click them to see all the other articles that are tagged with the same subject.
Read the article, and circle words that jump out as you as potentially significant - additional synonyms that researchers in this field use, or aspects of the topic that you didn't consider at first. Each time you find one of these terms, add it to your list.
This list becomes your jumping off point each time you approach a new database - it will help you make sure that you have been thorough in your searches.