I may be alone, but I don't particularly like when people knock on my door to sell me something. The way I see it, they've come on my property to bother me with something I probably don't want. I'm sure there are people that don't mind it, but unless you've got great selling skills, the return won't be that high. And you bring up the point about wanting the cars to be there. For most people this is the evening. What are most people doing at this time? Probably cooking dinner, doing laundry, watching a movie with their family. Not a time they want to be bothered. You could try the weekends though.
If you're looking to just do washing around your neighborhood, if you have a good high traffic road in front of your house, or anywhere you go to wash for that matter, you could put up a sign telling people what you do.
Could you let us know a little bit more about what services you plan to offer and how?
EDIT: PhaRO has some good ideas. I need to learn to type faster.