The best bit of advice I can give you for the aptitude test is to avoid flapping. It's easy to try too hard and make needless errors. Turn up switched on and ready to kick some ass, but don't dive in and start trying to achieve things that your brain isn't capable of coping with; it sounds kinda weird, if you haven't done it, but it makes sense when you're there. I wouldn't say relax, but its definitely more a case of easing up slightly, so you can work both quickly and accurately...sort of like less haste and more speed.
Good luck!