I've found a great tool for getting at those hard to reach places where road grime likes to get a death grip on the exhaust pipes. Bought a dremil "Carbon Steel" Wire wheel. Go easy, dont dig it in a burn the heck outta the chrome, it removes bits of caked on grime and very light rust spots like a dream. Areas like the weld seams, by the baffles and pass peg mounts seem to have junk in the nooks and crannies, this takes it out and leaves the chrome looking like new. Just go easy! After that give her a buff with your favorite chrome polish (I use "Mothers"). Looks like this when your done:
Set of ORIGINAL 1971 HM-300s. Chrome was beautiful when I started but had tons of grime caked everywhere, using a toothbrush and brake clean/carb clean didn't touch most of it.
Note the middle weld seam and Pass peg mount area on this pipe. It looked nasty when I started but it all turned out to just be road grime with good chrome underneath. No damage to the chrome at all, not even little marks like when you use OOO steel wool.
Hope it helps!