Chances are you don't, but you'd be surprised at how some people are washing their cars and how they're doing it wrong. First let's get over the basics.
Use a good car washing soap like Maguire's or Blue Coral. Do not use dishwashing liquid on your car! Dishwashing liquid is designed to disperse oil which is what your car wax acts as.
Take off all rings, bracelets, watches or any other jewelry to avoid scratching the car inadvertently.
Do not wash the car while the metal is hot or in direct sun if you can help it. This will lead to streaks and swirls.
Keep the car wet at all times if you are washing it in the sun. Even if you have already cleaned one whole side of the car make sure you go back and spray it to keep it wet. This will avoid water spots in case the water were to dry.
If you can - wash outside, dry inside
Wash the car using a wool mitt or a very soft cloth, nothing that will scratch the car. If using a mitt make sure you wash it before you start to clean the car in case there is dirt or stones left in it from the previous wash.