Josh, I used steelwool and some strong soap and they cleaned up great on my Limited. I think Brasso may work as well. After I cleaned them, I wasn't thrilled about a black round hole in a nice looking shiney chrome square pipe.

I degreased the inside of the tips, used high temp gloss black spray paint. Then I took a rag with some WD40 on it and wiped off any overspray on the chrome edge. I used gloss black because I wipe them out with spray wax when I wash the car. Flat black may look better, but harder to clean the exaust off of.

Good Luck, James