Talk to your professors and ask their advice on what literature you should read before taking the GRE subject test in English and literature. While there is no way to know beforehand what materials will be tested in the GRE, professors can guide you down the right path.
Read classic texts, including poems, on your own. This will supplement materials covered in your college classes. If you have not read works such as Bunyan's "Pilgrim's Progress" and Eliot's "Middlemarch" in class, read them at home. Milton's "Paradise Lost" is almost guaranteed to be mentioned in the test as well.
Find a study guide for the GRE subject test in English and literature online or at your local library. This guide will help you determine what works and time periods to study, as well as the various literary devices and terms you will be expected to know when taking the exam.
Take a practice test; many study guides will include these. Practice tests can assess what you already know and which areas you should focus your studies in.
Dedicate a notebook to taking detailed notes about the works you read at home. This will supplement class notes. By writing and studying both sets of notes, you will be well prepared for the exam.