The Institute of Medicine recommends in their publication Finding What Works in Health Care: Standards for Systematic Reviews, working with a librarian or other information specialist to plan out your search strategy and to peer-review the final strategy used: Standard 3.1.1 and 3.1.3.
Searching is a critical part of conducting the systematic review, as errors in the search process potentially result in a biased or otherwise incomplete evidence base. Searches for systematic reviews need to be constructed to maximise recall and deal effectively with a number of potentially biasing factors. (McGowan, 2005, p. 75)
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, 2008, p. 130)
A useful technique is to check the search strategies used in other systematic reviews for hints on terms and combinations to use.