Obviously my experience isn't completely representative, I don't really know many others who have applied for English and I myself am still at school. But here are a few tips I would give:
- Like others have said, find an author or a movement you are really interested in and really look into it in depth. This could involve looking at what influenced it, or what it has influenced. Reading critical essays and books can really broaden your perspective.
- Go beyond those generic reading lists, go below the surface and find something that's maybe a bit quirky, a bit off the beaten track. Pretty much every teenager with an interest in books has read things like Catcher in the Rye and Pride and Prejudice. Then again, if you really love one of the more "popular" books, try reading other works by that author, which are perhaps less well known. (I myself love Sylvia Plath, which isn't exactly unheard of for an academic teenage girl!)
- Try to have an idea where books fit in within the wider context of English Literature. Is it a part of a movement? Does it represent a reaction against what came before it? Historical knowledge can really enhance the way you approach a text; could the text be viewed as product of a specific time and place?
- Literary theory is a good thing to become acquainted with. The best quick introduction I can think of is "Doing English" by Robert Eaglestone, which introduces you to some of the key debates in literary theory.
- Extra-curricular, like many have said, are overrated if they are not relevant to the subject. Of course, if you enjoy something, don't stop it just because it won't help your application, just don't waste valuable lines on your personal statement.
This may all sound a bit mercenary, but if you enjoy and love English it really isn't. All my advice can be summed up with the words "read, read, read".
Sorry, if I'm repeating what everyone else has said and if you have any questions, just PM me!
Edit: I've just realised how long that post was, maybe I should get back to revision!