I served as the Director of Graduate Studies in the English department at Lehigh for several years. Over that time, I read hundreds of applications and advised dozens of my own students. Here are some tips for the Statement of Purpose, perhaps the most important document in the graduate school application.
Applicants will also want to be aware of the extreme challenges of the current academic job market for English Ph.D.s. These tips assume you already know how bad things are, and still want to give it a go.
General hint: this is a very challenging document to write for most applicants, and you should plan to do several drafts & versions. Start early! Preferably in the summer before the fall when you will be applying. Before you start to write, take some time to think about how you'd like to present yourself professionally and intellectually.
As you work on the document, you should also plan to get feedback from trusted friends (especially those who know how English graduate programs work) and faculty mentors. It is especially important that faculty mentors who will be writing your recommendation letters see your SOP, as they will inevitably be using that to shape their own letters on your behalf.
What is the Statement of Purpose?
Fundamentally: The SOP should:
1) describe your research interests, including traditional period and region interests,
2) give an overview of previous experience and background (courses taken, sampling of topics covered), and
3) identify theoretical investments.