I have used this one for years. Many people without realizing breathe very shallow. Breathe as you normally do and pay attention to how much you fill up your lungs. Don't breathe any different then normal. Many times when you consciously pay attention to it you will do it correctly. Not everyone, but a majority of people should notice that they are only filling up a small portion of their lungs. Increased breathing speeds increase your heart rate, which results in other factors. Our goal should be to have a relaxed steady heart beat which is a result of steady full breaths.
Sitting in a chair, in a proper position with your back straight do the following. In one count fill your entire lungs. The next one count let it all out and repeat. After doing this a number of times you will feel a little light headed. At that point you can stop. This is just to give you the feeling of a full breath. Make sure when you are breathing in you don't lift your shoulders up or do anything that causes tension in your upper body. You are to remain totally relaxed.
If you tend to breathe shallow it will take a while to instill the habit of a full breath. Don't rush it, eventually you will find yourself just doing it without thinking. Deep breathing exercises also aid in you entering a meditative state.
To begin with we will start with a 5 count, basically it is 5 seconds. I second per one count. What you do is totally exhale all the air from your lungs. Hold that for a 5 count then breathe in steadily for a 5 count. Make sure you don't fill your lungs up on the first count, on the 5th count is when your lungs should be full. Hold that for a 5 count and then exhale steadily for a 5 count. Repeat this as many times as possible. Make sure you are not lifting your shoulders up and down as you breathe in and out or anything else that would cause tension in your body. Once this becomes easy begin to mess around with the number of counts for each part.
Here are some examples:
Breathe in for 2 - Hold for 8 - Exhale 5 - Hold 4 - Repeat
Breathe in for 6 - Hold for 15 - Exhale 2 - Hold 6 - Repeat
Make up different number counts. Turn it into a challenge for yourself. Remember, if you are tensing up at all you are doing this incorrectly and practicing bad habits. It's much harder to break a bad habit and relearn a good habit then it is to just learn it right the first time so take your time.