Try not to take stress in exams. That is a major factor which makes you forget things. Also, keep your concepts clear and only mug up things if you have do. If you have no choice but to learn something (such as atomic masses of elements or any formulae) , do it the old fashioned way by writing the formulae again and again. Revise them in your mind throughout the day - while eating, before going to sleep, right after waking up, etc.