Kevin Haggerty wrote an informative article for The Chronical of Higher Education detailing how to be an effective reviewer. He encourages all academics to become reviewers because it keeps them abreast of new developments in their field, it allows them to shape the direction of their discipline, and it is part of their scholarly responsibility. Haggerty recommends that an academic reviews at least three manuscripts for every manuscript that he or she publishes. An effective peer review should have three sections: (i) a brief summary of the manuscript, (ii) comments about the overall quality of the manuscript, and (iii) comments about the particular issues. The article has many more details and is worth a quick look.