Get sunglasses with glass lenses. Glass is known not only for its outstanding optical properties, but also for the scratch-resistance. A lot of eyewear brands offer their signature glass lenses – Serengeti and Ray Ban are some of them. If you need vision correction, some eyeglasses frames can be fitted with Rx glass lenses as well. Though I would not recommend glass if your eyeglasses prescription is high as the lenses will be thick.
If glass isn’t an option, make sure that the lenses are treated with a special coating to prevent scratches. Some lens materials like polycarbonate have scratch-resistant coating built-in, some materials like CR-39 don’t. CR-39 requires an additional anti-scratch coating to be applied on top of the lens. If you buy sunglasses, you can check with a sales rep what the lens material is and if the lenses have anti-scratch protection. When buying prescription eyeglasses, you can request an additional anti-scratch coating.
Use a cotton lens cloth to clean the lenses. Avoid rubbing them with a paper towel as it can leave micro-scratches on a lens. Also try to keep your sunglasses in a protective case when you are not using them. A good quality pair of sunglasses or eyeglasses from a decent brand usually comes in a protective case or a pouch and is accompanied by a cleaning cloth.
If you take a good care of your eyewear, it will leave longer.