I love these forums <3In true TCS fashion, we have a whole 'dirty butt' thread
http://www.thecatsite.com/t/207425/solutions-to-dirty-butt
Not so sure about the original butter idea, but washing with a cloth soaked in warm water and trimming are both good.
I find that if I wet my cats anywhere, they just have to groom, so it's a good way to get him started.
Good luck!