Whether you're earning your turns on telemark boards, climbing manky rock and ice, or trying to get that perfect photo, if you need to drop a knee then you need to think about knee pads. Arc'Teryx makes a great pair in their Kneecaps. Basically a thin shield of stiff Kydex over a half-inch thick EV50 foam pad, the Kneecaps protect from any incidental contact you're likely to encounter in normal backcountry situations.
The foam is hollowed out for your knee, and the whole assembly flexes easily so your knees don't get any more workout than you intend. The T-lock buckles hold the Kneecaps in place securely and adjust easily to boot. At just eight ounces for the pair, it won't be weighing you down either.