Keep them in the original case. Indeed, even though Vuarnet mineral lenses resist scratches very well, your bunch of keys or a nail file in the bottom of your handbag risk damaging them with time. Result; not only are the scratches unsightly, but they also seriously affect your visual comfort.
When you take your glasses off, do so with both hands. In this way you protect the frame hinges.
When you put them down, fold them up and put them with the arms in contact with the surface; avoid putting your sunglasses down on the lenses, there's no better way to damage them.
Once back from the beach, simply rinse them under tepid water to remove sand and salt traces.