My missus has got a black car and I find these two things really help:
1) A really strong wax with good beading (I use collinite) helps each rain shower rinse the car down again. The heavy shower last night has made her car look freshly washed and its been 300 miles since I washed it.
2) After washing/drying or after waxing if your doing a "big wash", a spritz of quick detailer wiped off with a damp microfibre takes all the static off the car helping to keep pollen/dust off the car. Theres even a proper anti-static detailer out now (can't remember what its called).