The ways I have seen basically involve brute-forcing it, which is extremely time-consuming. I'm talking trillions of possibilities, and working through them one at a time via software designed for that purpose. There is no easy way that I know of. I have never succeeded, personally. The longest I let a brute forcer program work on a RAR was a week, 24 hours a day, then I gave up and deleted the RAR.
"Before you consider one an enemy, first walk a mile in his shoes. Then, if he *is* an enemy, you're a mile ahead, and you have his shoes."