First of all, let me state that the picture to the left shows what not to wear while washing your car unless you are expecting to use your cleavage to hold the sponge. And, if that's the case, you'll really want to make sure you are using a non-toxic, and non-irritating, soap to do the job.
Anyway, what's the most eco friendly way to wash your car? That is, assuming you still have a car and haven't switched completely over to public transportation or personal modes of transportation (like walking or biking)? There are a number of "eco friendly" products on the market that tout being low phosphorus and the like, but what's the environmental cost of using them?
The biggest issues at hand are those dealing with water usage for starters, but the most striking problem has more to do with what gets washed off your car and goes down the storm drain. In most areas, the water that goes into the storm drain (the drains on your street) doesn't get processed by your local, friendly waste water treatment plant. Most likely it drains directly into your local waterways, affecting fish and other wildlife populations with the soap, grit, grease, salt, oil and pollution that has been collecting on your vehicle as you drive it about town.
How can you mitigate the junk coming off your trunk? Look for a car wash that uses non-toxic soaps and phosphate-free, biodegradable detergents plus one that recycles its wash water. If you don't have a car wash like that in your area, you can get your car washed at a car wash that sends the spent water to the nearest waste water treatment plant for processing.
Another option if the above isn't available or you don't want to pay for it is to wash your car at home using one of the no or low phosphate soaps on the market. However, if you wash your car at home, do it on a flat grass or gravel area that will absorb the wash water before it can run into the storm drain.
If you want to try something quite fancy you can check out the environmentally friendly car washes that don't require any water at all. So, not only do you save money and resources on water, but there's no water runoff. You would just need to clean the cloth you use to wash the car, but the water used for that would go to your waste water treatment facility. I haven't tried these products yet, but The Lucky Earth Waterless Car Wash, Eco Touch Waterless Car Wash, Freedom Waterless Car Wash and Dri Wash are some to check out.
Of course, like many alternative methods, this one requires a lot more elbow (or cleavage?) grease. Finally, when in doubt, I generally just don't wash my car very frequently. It saves all sorts of time, energy, money and resources. Plus, I'm just too damn lazy.
How do you wash your car and how often?