Don't be afraid of a pressure washer. Stick with the green (wide angle) tip and keep the tip 18" away from the siding. As a cleaning solution I use liquid laundry detergent, about 1/4 cup to a gallon of water. Spray on the detergent, give it 5 minutes and rinse off. Don't let it dry on the siding.

If you have mildew, pre-treat the siding with a 25% bleach solution allowing 5 minutes for that to work. Then proceed with the detergent and rinse.

If there are spots that remain you can hand wash those, dot't try to force it clean with pressure or damage can result. Use self control and know when to say when with the power tool.

Thanks weed cutter. I may try your laundry detergent idea with just a regular hose-end sprayer. Is the laundry detergent a good choice because it does not produce a lot of bubbles?

Laundry detergent works, rinses well and is cheap. You can use special "house wash" bought for pressure washers but I have never found the magic in it. Hose end sprayer might do the trick. Spray straight at the siding, not up under it.