This is one of those tiny tips that can make a big difference. Whenever you eliminate an answer, draw a line through the whole thing — don’t just cross out the letter, and don’t just put an “x” next to it. Otherwise, it’s very easy for your eye to get distracted. You end up going back and looking at answers that you’ve already gotten rid of. You think you’ve eliminated them, but subconsciously you haven’t done so completely, and consequently there’s a much higher likelihood that sooner or later you’ll accidentally fill in a wrong answer.
There’s also a psychological aspect. While working through a question, I find it a relief to be able to eliminate an answer thoroughly. Out of sight, of mind — it’s just one less thing to deal with. It shows you in a very concrete way that you’re making progress, and that tends to boost your motivation.
Besides, it can be very satisfying psychologically to cross something out so completely — it’s like saying, “Take that, you stupid test!”