I have cedar siding on my house also. Just finished repainting a couple of week ago. Before painting I always wash the house to remove mildew and dirt. This is the procedure that I used.
First get a cheap pump up sprayer for your cleaning mixture. I got mine at a local Tru-Value store for $11.00. It holds about a gallon and a half. I also purchased a long handled brush, five gallons of clorox, and a box of TSP. TSP is tri-sodium-phosphate, a non sudsing detergent, and is located in the paint supplies. I also had my electric power washer but a hose with high pressure spray will work, just not as quick.
Mix the cleaning solution in the pump up sprayer with one half cup of TSP, three cups of clorox, and fill the remainder with hot water. The solution can be varied stronger or weaker depending on the stains.
Pump up the sprayer and soak the siding, let it set for about two or three minutes but do not let it dry. Then spray or hose it off. Repeat as necessary until siding is clean. If the stains are particularly bad scrub them with the brush. I wasted my money on the brush, only used it twice on the south side of the house. Also, the cleaning mixture is deadly to wasps. Or at least it stuns them enough you can step on them when they fall to the ground.
After I had cleaned my house it looked so much better that I would have put off painting for another year, but had already bought the paint. Which leads me to another story.
I have always used, and hated, a Wagner Power Painter to paint my house. This time I decided to brush and roll the paint on. The problem was that though the final color was much darker, when wet the new paint was the same color as the old. So after applying the first coat to the front we found many small places we had missed. I knew you can't spray latex in a regular siphon type sprayer but Sears had one with a pressurized can. Once I figured out how much to thin the latex it did a great job on the house, shop, and other buildings. I thinned it with two cups of water per gallon of paint.
I have used this cleaning formula for 20+ years and five paintings of my house.
Hope this helps.
Randy