I do a couple things that aren't recommended, but work for me. When the bikes are super clean, I'll apply ceramic wax(Griot's)...this really helps keep them free of stains. It specifically states not to use on matte finishes, but I haven't had issues on my matte bikes. YMMV on that though.

If I have sap or something greasy on the bike, I'll use alcohol to clean it. If it's really messed up, I'll spot treat with a magic eraser. Be careful with that though, as magic erasers are mildly abrasive...I haven't had issues with damaging paint finishes, but I use it sparingly.