You'll want to make good use of the syllabus and past papers.
Ration the past papers out - I would advise doing one (or two) at first, then the rest in the weeks leading up to the test. Don't forget to take the sample paper on the website too. The syllabus could contain some things you haven't done since GCSE, depending on your exam board. I would advise getting to the point where you could comfortably talk about *anything* on the list to a friend. This might seem like overkill, but for Oxford, the PAT is as important as the interviews, and you only get one shot - as opposed to the interviews, where you get two or three.
You'll notice there's a funny algebra question on each past papers, where you get several bits of information about boxes, or rubber ducks, or something. It's a good idea to prepare your method of answering these beforehand, by drawing a table or something.
Finally, the secret to doing well on the PAT that not everyone knows about is probably the British Physics Olympiad. I'd advise doing the
AS Challenge past papers and taking a look at the GCSE ones. They're quite similar to the PAT - in fact, during an open day, a professor told me that the PAT was originally based on the GCSE olympiad(!) The BPhO Paper 1 is the first A2 olympiad, and may be of some use if you're daring enough to look a little further ahead. The skill of checking work is something that can score you a lot of marks, so you could also practice doing AS Olympiad papers under timed conditions.