Custom made search filters (also called "hedges") can be very useful for searching on certain topics. For instance, In CINAHL, a search for "nursing education" with the geographical filter "Canada" returned 1576 records and the same search with "Canada" as a keyword rather than a geographical filter returned 1347 records, but the same search with this custom filter returned 2543 records.
Below are some excellent sources of custom filters. These custom filters are generally database specific, but can often be adapted to work in additional databases.