Hand Searching is a manual process of looking through journals page by page, often involving looking at each reference in each journal article found. Hand search to find studies that may not have been indexed in the databases.
You should hand search conference papers as well as the most relevant journals in your field.
Make sure you document the hand searching process.
Ask your librarian for assistance.
To learn more about why hand searching is so important, read this 2005 article from Armstrong et al.: It's in your hands: The value of hand searching...
Cochrane offers information on hand searching as well.