Before you start your review proper, it is sensible to develop a protocol that you will follow. This will ascertain what your review will include and what it may exclude. Developing a clear review question and protocol is the cornerstone of your systematic review, and will help you as you conduct your search and evaluate your results.
As part of your protocol you may have to do some simple scoping searches to discover the level of literature on a topic and to evaluate the appropriate limits that should be applied. You can also use these searches to inform the keywords you will use in your later searching.
Consider using text mining to help you come up with a list of keywords in a systematic way — for instance, PubReMiner is a useful tool, which carries out a basic search of the PubMed database and provides a range of analytics on the results. Keep your initial search fairly simple — you are coming up with ideas at this stage rather than carrying out your complete, systematic search.