Disclaimer: This is not from first hand experience, but from someone who has done the re-arching himself, my father.
You need a sledge (4 lb will do) and an anvil, piece of rail iron, or the like. You'll need to know approximately how much additional arch (lift) you want in the springs. Separate the spring into it's individual leafs. Trace the main leaf on the floor to give yourself a reference to work from. Now you'll want to strike the inside arch of the spring, the outside being against the anvil, working your way down the length. Try to keep centered. If you get off to the side, the spring will start to twist, but you can correct this by striking the opposite side of center from your last strike, or the one that started the twist. I would also put reference marks, say every 4 to 6 inches, down the inside so you could repeat it on the other spring. After each pass down the spring, compare it to your reference, and you'll see the difference. Once you get the desired arch on the main, do the same to the other leafs, matching them to the arch on the main.
I'm remember my dad doing this on his 66 CJ and he went too far, actually reversing his shackle angle. The more arch you put in them the stiffer they'll get. So don't over do it. And guarenteed, it's going to take some elbow grease. But it's the cheapest method I know.