Download the free practice booklet for the "Literature in English" GRE Subject Test from the Educational Testing Service (ETS), which is available as a .pdf file on their website, ets.org. Review the guidelines for how to study and prepare for taking the test. Get a feel for the full range of questions you'll have to answer by taking the practice test. Score your practice test using the guidelines and evaluate your performance using the criteria listed in the booklet.
Buy at least one study guide and create a feasible but thorough schedule for working your way through the book. Learn the test-taking strategies offered by the authors and practice what you've learned by taking sample tests, which the best study guides provide either in the back of the book or online.
Read, read, read. No amount of test-taking knowledge can make up for the vast amount of literary knowledge you'll need. Scan through the Norton Anthologies, review literary theory, study a literary time line--anything to increase your knowledge of the various genres and time periods covered on the test. Plan ahead: those who have not studied English or are not well-read might have trouble with the subject matter, no matter how adept they are at sitting for standardized tests.