I need to power wash my siding and I've never done it before - any helpful hints or dire warnings?

I’ve never used a power washer before. Should I borrow my sister’s dinky one or rent the real thing? Do I need to worry about damaging my siding? Is there some sort of special “wash” or do I make my own? I’m a babe in the woods, here (ok, the suburbs) and could use some help.