Specific to Broad: In thinking about your issue/topic, you'll want to first consider what questions you have. Many issues seem very specific, so it will help to try and think about the broader issue. For example, the issue of smells near the Butchertown meat processing plant could be related to neighborhood gentrification.
Good Keywords: Then, mine those questions for keywords to use in your search.
Keep Good Notes! Try to record your searches and the results (this is hard, but can prove a valuable time-saver if you can't do your searching all at one time).
More Keywords! Don't get locked into your first set of keywords. As you search, remember to look for additional words that might be used to describe the issue you're talking about. What are the ways professionals talk about your topic?
Evaluate as You Go: As you look through your results, consider point-of-view and authority. Ask from what point-of-view is the author of this (blog, magazine article, scholarly journal article, newspaper letter to the editor) writing? What is this person's authority to speak on this topic? (Is she an expert? Has he interviewed experts? Has she experienced the issue first-hand?, etc.)