Take advantage of the truncation symbol * -- the asterisk used at the end of a word tells the search engine you want all forms of that word.
For example, putting the * at the end of motivat* will search for motivate, motivating, and motivation all in one search.
This will result in a larger search result set...so if your search is already too big, this isn't a great tool to use. On the other hand, if you're not sure you're getting a good glimpse at all of the articles you should be, then doing a truncation search can help you see more!
Use search tips such as putting "quotation marks" around words you want kept together in your search. The databases will look for each word separately unless you tell them you want them searched as a phrase. Using phrase searching can help narrow and focus your results.
For example, you might try putting in the terms "childhood obesity" in quotes. This will ensure the words "childhood obesity" are next to each other when it searches the database which should result in a more focused search.
If words are typed into the search box in a string such as childhood obesity epidemic, the database would look for those words anywhere in the record. This is not always a poor option and is often a good way to begin a search, but sometimes putting quotation marks around the terms you want kept together helps narrow the search. So if you get several results on your initial search, think if there are terms that would make sense to group together using quotation marks to narrow the results. In this example, you could either use quotes around "childhood obesity" or "Obesity epidemic."
Keep in mind that if you do use quotes, put them around terms that make sense next to each other. If you put "epidemic child obesity" in quotes, the database needs to find those three words in a row. While that might happen in some cases, it will likely weed out several articles on your topic that could be useful.
Always take a few seconds to note the Subjects used on articles you like. These can be great terms to use to start another search or to try in another database. For example, if you typed the words childhood obesity in a search box and you found a fantastic article, you'd want to note that alternate subject terms used for that article are obesity and adolescents and you could use those Subject terms to search in another database. Alternatively trying a related term such as "weight control" and children could yield useful results. Just slightly altering your search strategy will bring back a whole host of other results.