its really easy mate
old sponge soapy water, getting rid of black dust in the tail pipe
brillo pad(yes you herd me)(the ones with the pink stuff in them) with your soapy water scrub away inside and outside, dont fret it wont scratch.
then dry off
using autosol, rub into all off the pipe inside and out
leave for the length of time it takes to clean the car
after that rub auto sol off
all of the above is my method for the last year and half, with these results
cheers
craigy