Read, read, read, read, read. Lots of history and historiography, lots of novels, poems, non-stop Shakespeare, and literary criticism. If you get in, you'll have three years of non-stop reading, so you might as well start now.
EH Carr, What is History? Marc Bloch, The Historian's Craft Fernand Braudel, On History Peter Geyl, Debates With Historians
Richard Evans wrote the introduction to the latest edition of Carr, and there are two volumes of essays by young historians responding to Carr.
Maybe look at some game-changing megabooks such as Braudel's Mediterranean, Keith Thomas' Religion and the Decline of Magic, EP Thompson's The Making of the English Working Class, and Niall Ferguson's The Pity Of War.
I also suggest Empireland, Longitude, and Stalingrad.
Skip Das Kapital and read instead The Eighteenth Brumaire of Louis Napoleon - Marxist history in a nutshell.
Have a look at this for some insight into the history of history at Oxford -
At least one work by each of Fielding (Tom Jones), Sterne (Tristram Shandy), Austen (Persuasion), Dickens (Great Expectations), Thackeray (Vanity Fair), Trollope (The Way We Live Now), Hardy (Far From The Madding Crowd), James (The Portrait of A Lady), Conrad (The Secret Agent), Joyce (Dubliners), Woolf (Mrs Dalloway) , Forster (A Passage toIndia), Waugh (Sword of Honour), Orwell (1984), Rushdie (Midnight's Children), Barnes (Flaubert'sParrot), Ali (Brick Lane).
Bits of Donne, Pope, Keats, Shelley, Wordsworth, Byron, Eliot, Larkin.
Shakespeare's Sonnets and as many plays as you can get through.
Katherine Rundell, Super-Infinite Roland Barthes, Mythologies Terry Eagleton, Literary Theory John Sutherland, Is Heathcliff a Murderer? and Can Jane Eyre Be Happy?
In addition to the great posts above, essay competitions can be a useful thing to enter and mention on a personal statement. And obviously doing well on the admissions tests is key!