Instead of performing searches in PubMed, for key words or subject headings, a search of the grey literature may involve first considering what types of organizations are likely to be interested in the same research questions you are investigating. Consider:
There are many sources that list grey literature producing organizations in the health sciences. Searching these lists for organizations relevant to your topic is a good way to start. Some of these sources are listed in the box to the right.
Once you have identified an organization of interest, you can generally locate publications on their Web site. This may be done by utilizing the site's search box. However, many organizations have specific sections of their site dedicated to publications (see the image below for an example). You may find what you are looking for more quickly by looking there first.
The strategy you use to locate literature on these sites will likely be broader than the strategy used to locate articles in a database. In systematic reviews, particularly, the strategy used in a database like Medline will be very narrow and focused. However, the strategies used to locate grey literature will need to be broader. For example, you may search for "clinical practice guidelines" in a database, but search just for "guidelines" when looking for grey literature.
These sites provide directories of organizations that produce grey literature in the health sciences. The Encyclopedia of Associations is a general resource for organzations which may be relevant to your search.