Once you have conducted your literature search, it is important to look back at the material you have retrieved and consider whether it meets your research needs - do you have enough material to support or refute your argument, do you have literature that discusses all aspects of your own research?  If your needs are not quite met, be prepared to start the literature search cycle again, re-visiting the planning stage, looking at your terminology to alter or expand it.  Consider using other databases or different filters within them. 

Try using different search techniques, for example, if you find one article that is particularly useful, look at the keywords used to describe it and try including combinations of those in your search.  Look at the references in the article and follow those up, again noting down any different keywords used.