The first thing I do before starting my search in a database is to brainstorm for keywords. Write them down as they occur to you. You may think you'll remember them all, but don't count on it!
Think of alternative words you can use to supplement your main topic. For instance, if you are looking for human behavior, you might also use the British spelling behaviour. Additional keywords might be attitudes, approaches, etc. to mean or imply the same. Then think of how you want to limit your main topic. What are your secondary keywords?
After you've put your thoughts together you want to find articles on the effects of advertisements on human behavior. Now you can put your keywords together. For example:
(human behavior or human behaviour or human attitudes) and (advertising or advertisements)
If you find this too complex, you can always shorten your search to a basic format:
human behavior and advertisements
After you view a few of the records, look at the subject headings for an interesting article and you may want to redo your search using those new keywords. Using the assigned keywords is always an easy & convenient way to expand your list of keywords without doing any real work!