Spray with mild detergent from a hose end sprayer while its hanging on the mast... (or a line to keep it off the ground if you have such at home...) Don't let it dry... let it soak a bit. Maybe brush with a terry cloth towel wrapped around a soft brush on a long stick. Rinse with plenty of water.
If it has mildew I recommend a product called Odo-Ban (mixed in the mild detergent above) to kill it. Anything that will remove the stains will damage the sail.
The higher quality the sail the harder to do more than just rinse off dirt without damaging it.
Old dacron sails you can clean better, getting a little more aggressive with scrubbing and somewhat stronger soap. You'll wash the treatment off the sail that makes it crisp but that can be reapplied. (to dacron.... like an OEM 1980's Sunfish sail )
Obviously you want to do it on a day with essentially no wind. bright but overcast helps you let the soap soak dirt out without drying.
Texas summer day 105 and bright sun... bad day to do it.
Generally if you hose the sail off to remove salt after it gets spray and such then let it dry... that is all you need if you handle the sail storage well. Make sure its dry when stored. The sail lives essentially on a clothesline when in use.