The aim of a search is to identify as many relevant studies as possible and to be as extensive as possible. For more information, see the Cochrane Handbook, Chapter 6.4.4: Sensitivity versus precision.
It is useful to see the Title Registration Form for a newly proposed review title in order to start working on identifying search terms.
Many CIS work with review authors as they are the subject expert and should be able to help with synonyms and alternative ways of describing interventions and conditions. They may be able to supply example reports of relevant studies you can use to identify relevant indexing terms in MeSH or Emtree.