If you just want them clean you can do what potassium suggested and use a tar remover followed with a metal polish and then maybe some kind of sealant.
If you want to go all out this is the proces I use:
Step 1: Fine grade Steel wool (0000).
Step 2: Fine grade sandpaper. Depending on how deep the micro scratches are. I usually start around 600 grit and work up.
Step 3: Apply metal polish by hand or with a tool similar to a Flitz. I use Mother's Billet polish
Step 4: A high heat resistant sealant.
EDIT:
Here's mine after. I did the inside as well. I didn't spend too much time since I wasn't looking for a mirror finish on the inside.