If you're doing English, History, Philosophy, any essay-based humanity, do all the essays you're set and get them marked! I kicked myself after exams for not taking the opportunity to get my essays marked, because for English and History, two suggested practice essays were actually in the exam!
Say you're writing an essay on Daisy for The Great Gatsby. Before you start the essay, use the internet to find 3 or 4 previous essays on Daisy (there are tons). Read critical material, watch lectures on that character, read other people's interpretations of her, and do all this before sitting down to write. Over AS you shouldn't get too many essays too soon and you can afford taking 2 hours or so to get a better overall appreciation of the essay topic. Save all the websites you've used for revision in May. Get into the habit of reading other people's opinions and thoughts and critically evaluating them ("Wilson Knight said X Y Z about this topic, and whilst I agree with his thoughts that the use of X Y Z shows this, I think actually X Y Z here could be a reference to X Y Z") because you'll get higher marks in essays, and you'll get into great critical thinking habits for A2 and university. But don't plagiarise!