Before the Dive - Masks
Scrub the lenses
During the manufacturing process, mask skirts are injected with a releasing agent that helps them pop out of their moulds. This chemical tends to leach out of the silicone and onto the mask lens, contaminating it.
To remove, mask-prep compounds are available, but any mild liquid abrasive like toothpaste will do the job. Rub the toothpaste into both sides of the lenses several times, rinsing thoroughly. It doesn't hurt to do the same with the skirt and strap assembly too.
Check the buckle system to make sure the buckles are functioning, then put on the mask for the initial strap adjustment. Ensure the headband portion of the strap is centred on the back of your head and pull tight as necessary, being careful not to pull the straps too tight.
Returning home after a days diving...
To make sure all grit and salt is removed, give your mask a good soak in fresh water. It's important to clean the lenses; sunscreen that can migrate onto the lenses will create fogging issues if not scrubbed off periodically. Not that that's a common issue for us Brits!
Once washed, look for cracks in the skirt or the frame near the face plate. Also inspect the strap; stretch it while looking for cracks or small tears. If ever in doubt about a straps reliability, replace it. Also check the buckle attachments for cracks or debris.
To avoid mildew growth, allow the mask and snorkel to drip dry completely before putting them away. Do not store them near light, heat or fumes. To protect the clear silicone, store masks and snorkels away from other dive gear - the darker pigmentations found in other equipment, especially neoprene, can cause the silicone to become discoloured. Pack the mask loosely, preferably in its original hard case, to avoid misshaping it.