Every time you pump gas. Throw away all your garbage while you wait. My car is filth free using this method.
only applies to chumps who have to pump their own gas, though
but my method is similar... I stop by a local bodega or Dunkin Donuts 2-4 times/week for coffee. whenever I do, any garbage in my car comes with me and gets throw into the trash in front of the store.
