Hmm. For maths, its not that hard to score A1. That said, I recognise that a lot of people's fortes isn't in arithmetic.
The trick is to practice. Most questions are quite straight forward, and probably would ask the same question, except using different numbers. Plenty of practice with maths would help you score. Try doing a lot of mental sums in real-life situation (I.E, trying to guess the amount of change that you're supposed to get if you bought something. Of course, before the answer is flashed to you at the counter). It would not be directly useful, but it keeps your brain alert and at least improves the mental calculations.
Yes, I realise a lot of students these days are spoilt because of the calculator. Simple addition/subtractions and they instinctively reach for the calculator
Of course, another trick is NOT to rush in the papers. Time yourself nicely so that you complete the paper with at least half hour to spare. Then, use that half hour to recheck every single question. A lot of careless mistakes are made when people rush, so... yea. Don't rush!!
Unless, you're seriously lagging behind time. Then, the trick is, don't panic!!