Checkout the Norton or Longman Anthologies of both British and American Literature. Read all the introductions to each time-period. These are very helpful. The Norton has an online version of their historical biographies as well as multiple choice quizzes and summaries that are extremely helpful! Remember that the GRE focuses much more heavily on British Lit, so plan accordingly. These can be very expensive, so get them used if you can! Older editions are fine, since the principal material does not change much. I personally prefer the Longman Anthologies.
The Norton Anthology of British Literature:
The Norton Anthology of English Literature, Volume 1: The Middle Ages through the Restoration and the Eighteenth Century (Norton Anthology of English Literature)
The Norton Anthology of English Literature, Volume 2: The Romantic Period through the Twentieth Century (Norton Anthology of English Literature)
The Norton Anthology of American Literature:
The Norton Anthology of American Literature, Package 1: Volumes A and B
The Norton Anthology of American Literature, Package 2: Volumes C, D, and E
The Norton Anthology of Theory and Criticism (especially useful for those who don't have a background in literary theory:
The Norton Anthology of Theory and Criticism
The Longman Anthologies of British Literature:
The Longman Anthology of British Literature, Volume I: The Middle Ages through The Eighteenth Century (4th Edition)
Longman Anthology of British Literature, Volume II, The (4th Edition)