Use quotes for author, title, or exact phrase
Use + to include a stop word (a, the, for), words that would otherwise be ignored
Use – to exclude results including the term: Halo –Xbox
Use OR to include either of your search terms: Incidence OR prevalence
Use Advanced Scholar Search to find specific citations
Google maintains an official Google Scholar blog, which highlights the latest and greatest tricks and features of Google Scholar. Here are the most recent posts to this blog:
Lister Hill Library
The results are relevancy ranked. Google Scholar explains that it "aims to rank documents the way researchers do, weighing the full text of each document, where it was published, who it was written by, as well as how often and how recently it has been cited in other scholarly literature."