I use 'white wall tire cleaner' from the auto parts store. Be warned, it says to use rubber gloves & protective glasses when using it... USE THEM!!!!!!!
My boss swears by Westley's Bleche-Wite for her white letter tires so I tried it on my cruiser tires and it worked better then anything I've tried. Just sprayed it on, hit it lightly with a scrub brush and rinsed them off and my tires were cleaner then they were new.
Picked up the large spray bottle at the local mega-mart for under $4