Check that the spelling and spacing is correct. Capital letters do not matter, as the search page is not case sensitive. Boolean operators such as AND, OR, NOT, (" ") phrase search or (*) wildcard search can be used to refine search results.
If too many results are returned, you could try refining your search by using multiple keywords together. This can be done by checking the tick-box options which appear on the left of the screen or by adding words to your original search.