Clean the lenses wet, never dry. Wiping dust off a dry lens grinds grit across the coating and scratches it. Rinse the sunglasses under cool water first, add a tiny drop of washing-up liquid if they're greasy, then dry with a clean microfibre cloth — never a shirt tail or paper tissue, which are abrasive and haze the surface over time.
Keep them off your head and out of the heat. Perching sunglasses on your head stretches the acetate frame and pushes the arms out of alignment; a case in the bag is kinder. Heat is the real enemy of acetate — a hot car dashboard or a sunny windowsill can warp the frame and loosen the lenses for good.
Tighten and adjust gently. The tiny hinge screws work loose with wear; a turn with a precision screwdriver every so often keeps the arms firm. If the frame sits crooked, warmed acetate flexes — an optician can re-true it with gentle heat, but forcing a cold frame is how they snap.
Store in a hard case. Fold the arms and keep the glasses in a rigid case whenever they're not on your face. It's the single thing that most protects the lens coating and the frame together, and it stops them being sat on at the bottom of a bag.