Loop a hair tie through your air vent. Stretch the hair tie away from the vent and slip the cell phone into the two loops.
Cupholder crumb catchers:
Place cupcake liners in cup holders to catch crumbs and other messes. When it gets dirty, just throw the wrapper away and replace with a new one.
Stick a suction cup shower caddy to you rear side window to store items like ice scrapers, maps, etc.
Cut an over-the-door shoe organizer to about the length of you back seat. Use string to attach the organizer to the back seat headrests.