How many days are left for the exam? If you have enough time, do like @Ivan, @Jeruba and @FireMadeFlesh said; understand it naturally, logically, derive them.
Write formulae down on a paper, and read those daily, hourly or after some time.
You can also try to remember them with some examples, e.g. nCr and nPr, take n=r=2, now think which one would be greater? There’s only one combination of 2, while you can arrange them in 2 ways. So, nPr must be greater than nCr. Now, to get nPr greater its denominator should be smaller. So, nPr=n!/(n-r)!, and nCr=n!/[r!(n-r)!].
Try memorize them, and write down all the formulae yourself, without any reference. Check for mistakes, and keep on repeating this till you get all correct answers and you’re confident enough. Yeah, repetition is the key as @Grisaille has said.
However, if you’re in hurry, (or just before exam) memorize whatever you can and after you get the answer sheet, don’t wait for the question paper, write all formulae on back paper with pencil (title it a rough paper). And, after you’ve used them, erase them out.