The best way to pack clothes for a long road trip is to keep it simple. Pack 5 to 7 days of clothes, then plan on stopping to do your laundry every 5 to 7 days. This will save you room in your suitcase and in your car. Use packing cubes inside your suitcases to keep yourself organized. Try bringing some bags that have soft sides, like duffle bags, this will help you pack your trunk more efficiently. It's amazing where you can squeeze a duffle bag into the car.