How do you revise for exams?

Last year I found it so easy, I could just study non stop. But now, for some reason I can't seem to concentrate, I am always looking for excuses to do something else.

I need 4 A's and a B. From the results of my prelims (2 A's, a C and 2 fails) I really need to make some changes, but I do not know where to start.

To revise I usually do 30 minutes of a past paper, 5 minute break then repeat. I listen to music to block out the sound of my noisy house.