I had the same problem. I called the local siding wash guy, and his 2 man crew power washed with their solution, scrubbed every inch with ladders and brushes on poles. Came out looking brand new.
He rinses it with a "sealer", as he calls it, which repels the water/dirt for a period of time...seems to work. They also squeegeed the windows when they were done.
After his crew did the big scrub, I periodically rinse the siding with my power washer (maybe twice a year?), and it seems to keep pretty clean...
It took his guys ~7 hours, and he charged me $500 cash. It was worth every cent. Those guys worked like dogs.