How can I prepare for the HAA?

The main way to prepare is to look at past papers, however there are not very many of these yet so it will not take you long to finish them all. If you want to prepare further then getting a book of psychometric tests and doing the verbal reasoning questions may be useful for the multiple choice section, although these questions are usually much easier than the ones on the HAA. It does help get you into the right mindset, though. You could also find sources online and have a go at analysing them in a general way, thinking especially about how they could be useful, and how valid you think they are. Look at sources from a period you have studied and see if you can find clues to things you already know about the period, which aren't directly stated in the source. If you can do that then you can probably make guesses about what might have been going on in sources from periods you haven't studied. The source section of the HAA is all about deducing things from sources anyway, so it's a good thing to have a practice at.