I always turn off all the vents and keep the windows rolled up when off the pavement (even if it's hot). Once I find my campsite, I lock the car and never open until exodus day. Stays pretty clean inside that way. Once on pavement again, I open the vents and turn on the a/c, a little poof of playa dust, and then clean air
I put it on recirculation. That way you still have AC going, but you don't suck in outside air. Most american cars go to recirculation if you put the AC to the 'max' setting.
The other thing I do is put some plastic sheets (like you use for painting) in the trunk and the seats where I load stuff. That pays off at the end of the week when you load your stuff that's coated in dust back into the car.
And +1 for not opening the car all week. Some people practically live out of their car, and that's cool, but you *will*
get tons more dust into it that way.