once a year I strip the whole mech down including chainrings, cassette, jockeys, chain etc and soak them in degreaser. give them a good clean, slap them back on and bingo - you hear the silence!
pain in the arse job but worth it after all the winter crap
and as CD says, you never eliminate all dirt but you can get rid of a load.
next is to buy a cheap ultrasonic bath to use the degreaser in - that should make cleaning easier.
OCD me?? 