get a quick detailer, something like meguiar's
Meguiar's Direct Quik Detailer® -
or some other similar product, i haven't washed my rover in over 4 months and it's still as clean as the day i washed it. I apply frequently, sometimes as often as 3-5 times per week, only takes ten minutes and you preserve the nice glistening shine from you last wash and wax. Of course it has to be relative clean to use the quick detailer, if you have large particle of dirt you're better off washing it completely or you risk scratching the paint and use micro fiber towels.