Shifu Kraig at Old Warrior Horse Kung Fu Academy often reminds his students that "the breath is the most important part of your taiji and qigong." The first lesson students are given is to breathe to the belly (i.e., dan tien) so that the abdomen rises and falls with the breath. This belly breathing is one of most healthful changes you can make and costs you nothing. We are returning to the relaxed, natural breathing that we did as an infant and very young child. Belly breathing helps to bring awareness to your body, massages the organs, and increases blood circulation and lung capacity. Because most of us have not been breathing to the belly for years, if not decades, we may have to relearn it. Just like our qigong and taiji, we don't want to push and force, but rather be gentle with ourselves as we gradually relax and deepen the breath. Breathe smoothly, slowly, and exhale completely. To get the feel of belly breathing, Shifu Kraig recommends lying on your back with your hands on your abdomen and then feeling the abdomen rise and fall as you breathe to the belly.

Here are some tips from Bruce Frantzis of Energy Arts:

When you inhale, feel your breath come into your nose, down your throat and into your lungs and belly. Let your belly muscles expand to move your belly forward.
When you exhale, let your belly return to its original position and relax. Do your best to completely relax your chest and not use any strength to puff it out when you breathe. You should have very little or no sense of air going into your chest.
First, practice by focusing on expanding and relaxing your lower belly. Your lower belly extends from slightly above the top of your pubic hair to your navel.
When you can expand and relax your lower belly comfortably, turn your attention to moving the middle belly when you breathe, which for most people is more difficult. The middle belly extends from your navel to just before your diaphragm. Placing your hands on the lower, middle and upper parts of your belly as you breathe will give you clear feedback as to whether and how much your belly is moving.
Finally, concentrate on expanding and relaxing your upper belly—your diaphragm and solar plexus—just underneath your lowest ribs. Try to have your diaphragm move downward as you inhale and upward as you exhale. This will help push air into the back and top of your lungs, parts that seldom get exercised. Eventually, you will be able to move all three parts of your belly in unison.