When Byron Nelson used to help me with my chipping, he would stress that the lower body has to be active, just like it is during the full swing. Your knees and hips have to turn a little bit. Your legs can't be stuck in concrete. I completely agree.
To help get your lower body involved, start with your left foot and left hip slightly open, your weight on your left foot, and your right knee leaning toward the ball. When you swing, kick in that right knee even more (below). This will help keep your lower body active, and you'll hit down on the ball to produce a quality chip shot.
How do you choose the right club when chipping? The farther the shot, the less loft you should use. Some tour players are great at using one wedge for all greenside shots, but they practice—a lot. Most of you will get the ball to the hole more often by changing clubs depending on the distance. Practice this way, too.