Best way: remove calipers from car. Plug the brake line hole with a silicon plug. Sandblast with a medium abrasive. Heat to 400 degrees for 30 minutes to complete degreasing. Treat with iron phosphate. Powdercoat with high-temp powder, or ceramic coat (and tumble later for high gloss). Rebuild the brake innards, then re-install.
Next Best way: Keep calipers on car. Buy a kit (like G2 or one of the many others), wire brush, treat, and paint.
Next best way: Wire brush them, give 'em a quick treatment with either naval jelly or 'rust converter' or similar, paint them with whatever spray can you can get your hands on.
I've done all three.... the quick and dirty ones with the $4 spraypaint look just as good as the done-right powdercoated ones. It all comes down to surface prep.