The less activity you have on the roof the better off for the life cycle of your asphalt shingles.
That being said, ABC Roofing Supply sells a product called "Shingle Shield" that I've used with varying degrees of success. It's is a concentrate that you mix with water and spray with a garden sprayer and rinse with a water hose. Avoid any pressure washer use on the roof, cause it will dislodge your shingle granules and shorten the lifespan of the roof. Repeated applications may be required.
There are Zinc strips that can be installed along the ridge and hips that help to some extent with keeping roofs clean to some extent. If you've ever noticed clean areas below a chimney, it's the wash off from the copper that creates an environment that is not conducive to mold, algae and organic growth.
Next time you have a roof installed, ask that an algae resistant shingle is installed. There are zinc granules incorporated in with the stone granules to help keep organic growth down. If you have a AR shingle, call the MFG and make a claim. They may pay to have a professional clean the roof.