I have the same problem with my boy Gino, but in his case, it's because he is long hair and cannot always ''manage himself''. I do keep his bum bum shaved to help him.
I believe there are specific wipes for cats that can be used to clean their faces, Perhaps they also make some for cleaning their bum bums... or maybe you can use the ones for the face on the bum bum.
Any pet product store should have them, and I'm sure they could inform you about the best kind of wipes to get.
I would AVOID using regular baby wipes that are made for human babies, as they may not be appropriate for cats.
hope this helps!
Cookie July 1st 1999 f/ RIP July 31st 2008 -cat
Tonali June 3rd 2007 f -cat
Elphin April 21st 2008 m -cat
Stibule(Gino,Airell,Katla,Rasta)June 6th 2009 f- cat
Gino Ferrero April 8th 2010 m -cat
Airell Awenn Awnfenn Ep Berrth April 8th 2010 f- cat
Katla Chilam Balam April 8th 2010 f- cat
Rasta Farian Marley April 9th 2010 m- cat
Chipcat-Filou found June 12th 2011 (Ocicat 6months old) f-cat
Jane the Snow Cat rescued Dec27th 2012 f- cat