I have a cedar home and have been researching the same issues. This is what I have found. If there is still good stain on the siding then a simple mixture of water and dishwashing detergent may do the job before re-staining. Let the wood dry a couple of days before re-staining. If the stain or other coating is worn away, then it may be necessary to invest in a commercial cleaner that refurbishes the surface and takes away the mildew. Be careful using a power washer on cedar siding. It can very easily gouge the surface and eat away the soft part of the wood. Everything I have read recommends scrubbing on cedar versus a power washer. After cleaning, use an acrylic stain if possible to allow moiture to get through the wood. This may depend on what surface coating was used before. If you are working with bare cedar, use an oil based primer with an acrylic stain over that. The more solid the stain, the longer it will protect the wood from sun and weather. I imaging people paint over cedar but it seems like a waste if you want the wood grain to show. I have decided to go with a solid color stain to get the best of both worlds, however, it's a trade off. Paint will probably last a few years longer.