Manufacturers coat new masks with a protective chemical that you need to scrub off or you won't be able to defog the mask. To remove the film, use a soft cloth to gently scour the glass inside and out with a non-gel toothpaste or other low abrasion cleaner with fine grit that can remove the film without scratching the glass.
You should then rinse the mask thoroughly, with a tiny drop of household washing up liquid. Before going in the water, you should spray your mask with a defog spray to prevent it clouding up, alternatively a bit of spit will do just fine.
A mask should be rinsed with fresh water after every use, even if just in a pool. Masks usually last a long time but remember you will need to replace your equipment more frequently if using in the sea, the salt water is much more abrasive than fresh water that is encountered in a lake or pool. This means that the equipment will become worn and damaged faster, cleaning the equipment can help it to last a little longer.
Cleaning a mask is not highly technical, simple rinse thoroughly with fresh water after use, regardless of the type of water you used it in. If the mask was used in salt water you may find it more beneficial to leave it in a fresh water tank (many popular diving and snorkelling locations have these provided), but be careful about leaving it or someone may drop heavy equipment on your mask and damage it.
Then every four or five uses clean the inside of your mask with toothpaste. Apply the toothpaste to a soft cloth and rub it into the lens, skirt and corners of the mask, then rinse with fresh water and wipe. This prevents the build-up of oils inside the mask and helps to keep the materials in better condition.
Check your mask before every use. If the strap is frayed (fabric straps) or showing signs of cracking (plastic strap) then see to this before you next use it. A strap is the easiest part of the mask to get replaced and you should be able to get a new one without replacing the entire mask, you will need to check for brand compatibility however.
Check the skirt for any sign of tears and inspect the buckles, be sure to test your ability to use them even when wearing gloves. If anything appears to be damaged it should be replaced before it is used, again, most times the buckles can be replaced without needing to replace the whole mask.
Be careful about how you set your mask down to avoid scratches, as a general rule the mask should not be set face down. However, storing it in a bag or hanging up will end to prevent scratches or other such damage to lenses. Just be careful that the way you hand it is not going to damage the strap.
Masks should be kept in a hard case when not being used for any period of time; this case should ideally be a perfect fit for the mask and one is often supplied at the time of purchase. If the mask is in a compressed position it could become distorted.
Store them in a cool dry place away from direct sunlight. Also, if you have any neoprene rubber equipment, store your snorkel away from it as the neoprene dyes can stain and discolour any silicone parts. Whilst this won't affect the performance of you mask, it can make them look unpleasant.