If you do it at all,
use a ph neutral mild soap like Dreft. Soak the sail at least 12 hours. Do not wash it in a machine, use a large tub and knead it gently every so often. This usually won't take out serious stains, but it will clean off the surface dirt. Don't hang the sail to dry, lay it out flat, but allow for air to circulate under it. The stronger the cleaning agent you use, the harder it is on the sail (removing resin and degrading stitching and cloth). SailCare will dry clean it and take out most of the dirt, but even that won't always remove deep, ground in dirt or stains. They will treat it to replace the resin, but they can't replace the shape it had when it was new, so don't expect it to shape out any better than when it was dirty.
