In my experience, sails are pretty durable and don't really require a whole lot of "maintenance" but there are definitely some things to avoid. All modern cloths and threads are synthetic, so rotting is generally not an issue. If the sails are stored damp for a short period, it won't really hurt the cloths although they could develop mildew and the windows could fog. However, any aluminum fittings (headboard, clew) will eventually develop corrosion if they're stored in a wet, salty environment for a long period.
UV exposure is definitely a killer for many sails (although some new laminates are bonded with UV inhibiting adhesives). Of course when you're sailing, they're going to be exposed, but don't leave the sails out for long periods if you aren't using them.
Watch for abrasion points on the boat. Also don't roll up your sail on a rough surface like a parking lot. Doing so will wear your sails quickly.
Don't let your sails flog unnecessarily.
Roll your sails for storage. Try not to let them crumple or fold over on themselves and don't put heavy objects on top of the roll or you can put permanent creases in the sail material.

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