The Relaxation Response is the opposite of the Stress Response and is essential in counteracting the problematic stress in your life. Each of these responses are parts of the Autonomic Nervous System; the Parasympathetic (relaxation) and the Sympathetic (increased activity, tension).
One of the most effective ways to elicit the Relaxation Response is to learn to breathe abdominally. Start by lying down on you back with your legs bent and your feet flat on the floor. Put one hand on your chest and one hand on your abdomen and inhale slowly allowing your abdomen to expand outward. It is helpful to imagine your stomach expanding like a balloon as you breathe in. Obviously, you could just push your stomach muscles out which would look the same, but you would not be breathing abdominally and you would not be using your diaphragm.
Start your practice by spending 5-10 minutes a day breathing abdominally and gradually increase the time as you become more comfortable with the process. Work up to 20 minutes at least once a day. Once you can maintain the breathing for that period of time you will notice just taking 3 or 4 deep breaths will bring on the relaxation response.
The diaphragm moves downward when you inhale, so the stomach muscles will move outward. The diaphragm relaxes and moves upward on the exhale, you don't have to "force the air out," the diaphragm does that for you naturally.
There is another aspect to abdominal breathing which you want to pay attention to; breathe slowly. Normal breathing is somewhere between 12 and 16 breaths per minute; hyperventilation can be 25 or even 40 breaths per minute. When doing abdominal breathing your goal is to decrease your breathing rate to 2 or 3 breaths per minute. But initially just focus on just getting the technique down. Your first goal is to slow your breathing down to somewhere between 6 and 10 breaths per minute.
So, when you breathe abdominally, consciously slow down your breathing, if you don't you will almost be hyperventilating since you get 10 times the amount of air in your lungs when breathing abdominally as compared to breathing thoracically (chest only).
After you breathe abdominally for a few minutes your body starts to respond, starts to relax--let that happen. Don't work at it just let it happen. In the beginning some people have the sensation that they are not getting enough air and feel compelled to take a deep upper chest breath like a "sigh." The sensation of "not getting enough air" seems to be related to a change of physiology, from tense to relaxed.
If you feel compelled to take a deep breath, it is an indication your normal mode of breathing may approximate hyperventilation. As best you can, resist the temptation to do this. In time, with practice, this sensation will pass. And...rest assured, you are getting enough air. In fact, when you breathe abdominally you bring CONSIDERABLY more air into your lungs than when you breathe only with your chest.
Unless you are a singer or musician, this type of breathing may feel strange at first. This simply means it is new to you, so be willing to give yourself some time to become accustomed to the sensations. Some concentration is required, and as you breathe abdominally you will feel some physiologic changes, e.g. heaviness in the body, drowsiness, and calmness. All these sensations will increase as you practice relaxation and by focusing your mind on them you can encourage these sensations and the relaxation they represent.