Things that increase your cars drag.
Weather Shields
High Rear Spoilers
Rear spoilers often increase downforce which is beneficial for racing, but come at a cost in increased drag.
Mudflaps
Your roof racks also add to drag so take them off when you're not using them
Also bad for aerodynamic drag is open windows, open sunroofs, radio aerials, side mirrors, large radiator/brake intakes, bonnet scoops and windscreen wipers .