My solution after losing many expensive pairs of sunglasses over the years was to buy two pairs of relatively cheap but serviceable ones. Needless to say, I have never lost them, although I have temporarily mislaid them but they have always turned up like the proverbial bad penny. The only other pair I managed to keep for a few years were a gift from someone whose friendship I valued greatly at the time, and thus they had sentimental value so I rarely wore them!