Bibliographic databases: When searching for a systematic review it is important that you search across a range of databases, as no one database covers all the health-related literature. The decision regarding which databases to search depends largely upon the topic of the review. Searches of bibliographic databases usually identify the bulk of the literature to be reviewed.
Grey Literature: Includes includes a wide range of resources. Grey lit may be ephemeral (that is, of questionable relevance or quality), but it continues to have an impact.
Handsearching: Involves page-by-page examination of relevant journal issues, conference proceedings and other publications for relevant studies.
Citation Indexes: Track references authors include in the reference lists of their publications.
You should aim to be as extensive as possible when conducting searches for systematic reviews. However, it may be necessary to strike a balance between the sensitivity and precision of your search.
Sensitivity - the number of relevant results identified divided by the total number of relevant results in existence.
Precision - the number of relevant results identified divided by the total number of results identified.
Increasing the comprehensiveness of a search will reduce its precision and will retrieve more non-relevant results. However,
"... at a conservatively-estimated reading rate of two abstracts per minute, the results of a database search can be ‘scanread’ at the rate of 120 per hour (or approximately 1000 over an 8-hour period), so the high yield and low precision associated with systematic review searching is not as daunting as it might at first appear in comparison with the total time to be invested in the review. "
Cochrane Handbook for Systematic Reviews of Interventions, 2011, Chapter 6.4.4)