Jumping and landing is the first building block of Parkour training. Good landing technique is the key to safety, protecting your joints and controlling of your movements. It is the first thing you need to learn, and yet one of the hardest to truly master.
Here's the basic idea: your body is a spring. Your only point of contact with the ground are the balls of your feet (the widest part, between the toes and the arch), everything else is springs, tensing and releasing in the jump and coiling back in the landing. Your ankles, knees and hips all work together against your mortal enemy, gravity. To improve your landing form (and your jumping form as a side effect), you must understand what each of these can and should do, and what they cannot.
The ankle controls the angle with which you reach the ground, protecting your heel from impact. It has a limited range, and is not very strong, thus you need to train it carefully and often. Upon landing, you will need to extend the ankle in order to reach to the ground with your toes before the weight of your body comes down toward it, and resist its inflection as the first step of absorbing momentum.
The knee is in the middle of the leg, and a major part of the coil, but not the driving part of the spring. The bending at the hips is what brings power to the jump and absorbs momentum on landings. The knee mostly provide a way to keep the lower leg straight and stable, but one can easily hurt it with impact (from the heels hitting the ground), over-bending (when it goes forward of your toes, bearing your body's weight and momentum) and twisting (if landing with any sideways momentum). Like the ankle, the knee can help resist the pull of gravity if the legs are mostly extended (not locked, though!), reaching toward the ground upon landing, and ready to slowly coil back in. But like the ankle, it is not strong enough to absorb fully the impact of the fall.
The hips are at the middle of the whole body, close to the center of gravity. They give power to the jump through tightening of the glut-hamstring chain (back muscle of the upper legs and butt, much stronger than the quadriceps in front), and slow the landing with a progressive release of the same muscle (really "fighting back" gravity). They also provide the needed adjustments to balance the weight of your body and reduce its load on the legs. This joint is not used enough in daily life, and needs some work to get stronger and reach a good range of motion.
Here's a few simple exercises that will be (hopefully) beneficial to this ankle-knee-hip chain. You can work on them often, however remember to warm up the joints first.
Cat paw walking: walk without touching your heels to the ground, trying to be a silent as possible. Explore different paces, all the way to light running.
Squats: really focus on leading with the butt, sinking down and pulling back up. And keep those knees always, always behind your toes. Do 20 of those every morning!
Squatting low: rather than sitting on the ground or your couch, squat as low as you can and relax into that position. You can read a book, watch tv, play video games... maintaining that position a bit longer every day will help improve your hip flexibility and increase your range of motion. Especially important for men!