Looking after your sails is often a matter of common sense. All sails wear with use but you can minimize this. For example why use your new set of racing sails for cruising around with the kids ? Many sailors continue to use an old set of sails for 'pottering about' whilst retaining a new set for racing. Also when you get your dinghy ashore don't let the sails flap about in the wind, get them down as soon as possible and try to avoid creasing or folding them.
UV light can deteriorate sails. You can minimize this by packing them away at the earliest opportunity.
If you sail in salt water then make sure you wash the sails in fresh water after use. Salt actually holds moisture so the sails never completely dry. If you need to wash sails then avoid using strong detergents as these can degrade the coatings and glue used on some sails. There are some specialised sail cleansers on the market so try to use one of them. Some sails such as those made of kevlar are often glued together and detergents can often weaken this glue eventually causing it to fail.
The best thing to do is try and ensure your sails don't get dirty in the first place! Try to avoid laying them on the ground if at all possible. Obviously there are times when this has to be done when packing the sails up ready for storage but try and do it on clean dry ground.
Firstly never store sails wet. Allow them to dry thoroughly. You should roll, not fold, sails. One of the best ways to store sails is to roll the jib up with the main sails to create a nice stiff tube meaning they are stronger and less likely to get crushed. The sails should be rolled from the head which ensures that you never need to crease the sail.Rolling also ensures that any plastic windows are protected too.