1) Take two buckets to the wash: One bucket for rinsing out your sponge, and the other filled with soapy water to wash your car with. Because if you rinse and wash in the same bucket, all you are going to do is end up cleaning your car with dirty water. It's a simple concept and an obvious one but one all too easily missed!
2) Wash the wheels first: Because the last thing you want to do after finishing cleaning your bodywork is to then splash dirt all over it when you go to clean your wheels. The wheels are one of the dirtiest areas on your car so wash them first then you can get more clean water for the rest of the car wash!
3) Work from the top down: This is another simple, obvious tip that is often ignored. Working down from the top ensures that you can rinse the car without getting soapy water on areas you have just cleaned and, seeing as the bottom of the car is often the dirtiest, it prevents you from spreading that filth and dirt all across the rest of the vehicle! Also, when you wash the windows, try to use vertical strokes on the interior of the windows, and horizontal strokes on the exterior of the windows. This way if you get streaks, you know what side they are on!
4) Use the right soap: You may think it is okay to just water down some dish washing liquid soap, but car soap contains ingredients engineered to help clean without damaging your paintwork. Using other soaps not designed for this purpose can compromise your paint and cause lasting damage!
5) Clean regularly: Yes, I know it's a horrible job and it takes time to do and do it well, but regular cleaning WILL make your car easier to clean next time around as there will be less dirt for you to remove. Regular car washing with good quality car soap will help to preserve your paint to keep your car looking (almost) as good as new for much, much longer.