Resist the first wash. Raw denim develops its fades — the honeycombs behind the knees, the whiskers at the hips — from where your body creases it before any indigo is washed out. Give a new pair a few months of wear before the first wash and those marks become permanent and personal.
Wash cold, inside out, and rarely. When it genuinely needs it, turn the jeans inside out and run a cold, gentle cycle with a little dark-wash detergent, or fill a bath and soak them flat. Cold water and no agitation is what keeps the deep indigo and black from going grey and patchy.
Hang to dry, never tumble. Heat is what kills denim — it shrinks the cotton, cracks the surface and dulls the colour. Line dry away from direct sun. If they feel stiff afterwards, a few minutes of wear softens them back up.
Spot-clean between washes. A damp cloth on a spill, a stiff brush on dried mud, and an occasional airing on the line will carry a pair a long way. Dark denim jackets follow all the same rules — wash them together and they'll fade in step with the jeans.