Try do to puzzles that are an above average difficulty. This a great way to train your mind to put things together orderly and quickly. Make sure you do a different puzzle each time, so you are thinking on your feet.
Try to solve complex math problems. This will help to improve the way you analyze things and can be tested since there is always a definite solution. Try doing three or four problems at a time before seeing if your correct, if you are not right, try to do the problems again and again until you get the right answer.
Do some memory exercises such as turning playing cards up side down and then turning them face up two cards at a time, then try to match the cards. Another great exercise is to write down fifteen words and try to memorize them, go on about your day and wait until you have forgot about the test, later sit down and try to write down all fifteen words on another piece of paper, then check to see how well you did.
Finally make sure to get plenty of rest. The mind needs at least eight hours of sleep to work at full capacity. Try these exercises as often as you can, who knows, you could be the next Einstein.