It's some time since I've done the AO / 3rd Sec exams, but I can offer the folowing, if it's of any use:
(1) Dereko1969 is correct - on the "maths" section (it's really only arithmetic, and a little data manipulation, at most), all you need is a pass. You will not receive additional points for further corrrect answers.
(2) There is no negative marking.
(3) There will usually be a good deal of information provided in graph and / or tabular format. Look carefully at the figures and what they represent - you may be asked to compare two things and assess which is better value, that sort of thing.
In general, they tend to throw in a couple of questions with a twist, or which require more calculation: if you're not quick with numbers, don't waste time trying to work those out at the beginning. As dereko says, identify the easy ones and do those. Keep an eye on the time. When you've done the easy ones, go back to the others and spend a little extra time trying to work them out. If you run short of time, pick the one that looks right. And try to save two minutes or so to look over your answers.
http://www.thatquiz.com/ has some online arithmetic and graph stuff - you can change the difficulty levels. While it's not exactly the kind of thing you'll face, it's good for sharpening you up if you're a bit slower on the numbers side, as are IQ quiz books and the like.
sun_sparks, I'm not sure what the numbers are like these days, but there used to be thousands, with panels of twenty to fifty being established. That would indicate to me that in the region of 300 people in total, at most, will be called to interview from the test stage.
Note that certain AO posts are being recruited directly by the relevant departments, so keep an eye on as well.
Best of luck to all of you taking the exams!