I don't understand why this is alway a questions. There are not that many PMP formulas and they are easy to remember.
If you can't remember the the PMP formulas then you don't deserve to be a project manager, let alone a PMP.
Is remembering how to calculate the SPI, CPI, BAC, BCWP, BCWA, etc... that hard?
In any case, a great way of doing this is through solving a lot of exercises, as many as you can. And then by writing all the formulas on a small cheat sheet that you can read every night before you sleep. Keep in mind that you cannot take the cheat sheet with you to the exam (it'll be considered cheating and you'll be banned from taking the PMP exam, possibly forever).