The landing is the most important skill to master in Parkour. A safe, light, quiet landing will keep your joints healthy and allow you to practice Parkour for as long as you want. Landing with deficient form can lead to all sorts of overuse injuries from tendonitis in the knees to low back pain and ankle problems.
Practice a good landing from different heights. Begin on the ground and do a Tuck Jump 25 times. From there, do 10 landings from a slightly taller height, maybe 2 feet and continue in incriments of 6 inches. If the landing feels at all hard or uncomfortable, go back to the previous height. For an example of landing form check out this entry in the Parkour Dictionary.
Conclude with a stretch, especially shins/ankles, hamstrings, and quads.