You have to experiment with different search strategies when you are doing research. Sorry, there's just no way around it. Boolean Operators can help you save time because they can improve your search results, but it's important to know when and why to use each one.
For example, let's use the research question "Do soft drinks contribute to childhood obesity?" Most databases give you the option to select Boolean Operators on the search screen, so all you have to do is select the operator you want from a drop down box between search boxes.
Here's our advice:
AND
Use AND when your research topic has more than one key term that must be present in all of your search results. In this example, use it to link the key terms "soft drinks" and "childhood obesity" so that both concepts appear in all of your search results. Searching with AND gives you fewer results.
OR
Use OR when your key term has synonyms which different authors may use interchangeably. In this example, use it to link the synonyms "soft drinks" and "soda" so that you'll get results with either concept. Searching with OR gives you more results.
NOT
Use NOT when your research topic is often closely associated with another concept you do not want information about. In this example if you are researching soft drinks and childhood obesity, but do not want information about vending machines in schools link the key term "vending machines" to the other key terms with NOT so that you eliminate all results that include the unwated concept. Searching with NOT gives you fewer results.