My mechanic sprays oil under the car..
In spring you can scrape the oil off and all the dirt would come down in a big bunch.
Just don't overdo it as you might start getting drips from all over the place!
I will do this soon...
Maybe you can get an air compressor and attach a tube filled with engine oil. Used engine oil should be fine. Maybe nothing from a diesel car
hehehe
good luck,
I wash my car often but from now on (since school just started the car probably won't see daylight until it's nice and sunny like today)