Brush up on your grammar. There won't be a lot of grammar questions, but the ones that do appear are intended to be free points. They should be the easiest questions on the test. What's a gerund? If you don't know, find a introductory grammar text book and look it over.
Study literary terms, such as caesura, alexandrine, and pathetic fallacy. I strongly suggest that you look into a book such as "A Glossary of Literary Terms" by M.H. Abrams. While not making up a majority of the questions you'll face, like grammar questions, they should be automatic points. Also, knowing these terms will help you correctly answer other questions on the test that aren't directly related to literary terms. For instance, knowing what an alexandrine is will help you identify poems by Spenser.
Read the works and authors that are most likely to be on the test and have the most questions dedicated to them. This includes:John Milton, "Paradise Lost"Chaucer, "Canterbury Tales"Shakespeare, "King Lear"Alexander Pope, "Rape of the Lock" and "The Tempest""Beowulf"Swift, "Gulliver's Travels"Homer, "The Odyssey"
Buy an anthology of English literature. Familiarize yourself with the authors and works listed. The GRE tests you on your knowledge of works and authors that the average undergraduate student in English is likely to have encountered. Therefore, a great deal of questions will likely come from such an anthology, especially a Norton version.
Look over the Bible. While the GRE subject test in Literature is not by any means a religious exam, many authors and works make allusions to famous lines, passages, and characters in the Bible.
Get your hands on several practice copies of the exam. These can be found in the numerous study guides at bookstores, online, and from the GRE itself.
Lock yourself into a room with a pencil, clock, and the practice test. Turn off your phone and make sure to tell people not to interrupt you. Don't take bathroom breaks, snack or drink. None of these things are allowed during the actual test and it is to your benefit to replicate the actual exam conditions as best as possible.
Take the test and try to finish as much as you can in 2 1/2 hours. Don't look at the answers or take any breaks until your time is up. Repeat this several times, even if it means taking the exact same practice test more than once. Familiarizing yourself with the procedures and the various restraints will actually help you improve your score.