A foam lance might be a good idea, I have this and it's great, especially when you consider the price.
http://************/38ypu2x
Although, I'm using a gas power washer running at ~3000 psi so YMMV if you're using a smaller one. Once you get everything set up, you can soap up the whole car, wash with a mitt, and rinse in well under 30 mins. When you include the setup though it's a bit longer.
You could probably use a foam gun instead (ie one that attaches to a hose rather than a power washer). It takes out some of the setup time but the pressure is obviously much weaker than what you'll get out of the power washer.