Nail the technique. I found that History was 40% subject knowledge and 60% writing and analytical skills. In terms of preparing for it, reading books and watching documentaries is really helpful, as not only will you passively retain bits of subject knowledge but you might also begin to understand how to formulate and present your arguments. It doesn't even have to be about the periods you're going to be studying (although that would certain help more), I think the key to success is to enjoy the subject. It is so much harder to motivate yourself to learn content and do practice if you detest the subject, on the flip side if you enjoy it then you'll be willing to go the extra mile.