Do you have a pile of some material at home that seems like it could be useful? Paper, leather, metal, cloth, etc? Give it to Scroungers' Center for Re-Usable Art Parts (SCRAP) and let them sort it out.
I found out about this organization from a neighbor who took a hundred Altiod tins off my hands and brought them over to SCRAP. Maybe you'll see an article about these folks in one of the San Francisco newsweeklies sometime soon.
If you are going to clean concrete, the 2,000 PSI washers you can rent are crap. Start with the 3,000 PSI washer and the 15-20 degree nozzle. If this cleans well, drop down to a wider nozzle. If things are going slow, break out the 0 degree nozzle and watch out!
If you are washing a house, a deck, or something that you don't want to strip the paint off, the 2,000 PSI power washer is probably plenty. I blasted out some older wooden decking with a 3,000 PSI model and a narrow nozzle. The soft wood between the grain was just gone. In an instant! Ooops.
You'll want to wear at least waterproof boots and foul-weather pants. A jacket helps if it is windy.
By the way, Action Equipment Rentals on South Van Ness is pretty good place to get this sort of thing. You can also pick up a tasty Mission burrito after dropping off the gear to boot.