With most sunglasses, it's the lenses that usually go first. They scratch, chip, and crack. Thankfully, you can now order replacements through Revant Optics ($24-$60).
The company's database covers 60,000 of the most popular shades: all you have to do is pop the old lenses out and pop the new ones in. For obscure or truly vintage shades, Revant will cut custom lenses for $44.