Walk backwards - IMO, walking backwards is the most effective way protect your knees -- though it's often the least practical and most dangerous approach. Walking backwards significantly reduces the strain on the knees during the descent, probably because walking backwards allows the knee to maintain a better ergo-dynamic angle during the descent. Walking backwards also shift effort away from a tired group of muscles. When it's safe to do so, I always descend backwards.
Take small steps - When walking forwards down hill, take short steps or "baby steps" as you descend. Short steps reduce the vertical distance traveled by the leg, which seems to reduce the impact on the knee.
Zig-zag down a hill - Instead of hiking straight down a hill, follow a zig-zag or switch-back pattern as you descend. This practice can decrease the pressure on your knee joints.