Remember - this is an iterative process: As searches are performed and familiarity with the topic and database increase, new keywords will become obvious. These words will be in titles, subject words, abstracts, and the full-text of the publication. Take advantage of them. Change your search strategy to incorporate new words.
Control your searches by specifying parameters (date range, format, material type, language,location) and dictating where your search terms should appear in an article (title, subject, abstract). These options help eliminate records that are outside the selected limits and increase the relevancy of your results.
Finding articles and dissertations on a relevant topic will help you understand the application of specific methodologies and provide justification for using a particular methodology in your proposal.