God grant me serenity to accept the things I cannot change (exhale), courage to change the things that I can (inhale), and the wisdom to know the difference (hold).  

With a regular breathing practice you can alter the blood chemistry and literally change who you are.  When we are stressed and the mind is agitated, we take short, shallow breaths and the blood becomes too oxygentated from over breathing and hyperventilating.  This causes other problems, such as more stress, asthma, metabolic and hormonal imbalances, allergies, muscular pain and an intolerance for carbon dioxide.  The body becomes too oxidized, and oxygen is a kind of poison.  You don't want more it, life-sustaining though it may be.

By slowing the breath down you are able to change the acidity of your blood by increasing carbon dioxide and your tolerance for it.  Paradoxically, that leads to greater states of relaxation and concentration than over-breathing.  So, it is a kind of paradox that in order to learn how to breathe better you need to learn how to breathe less.  You can teach your body to be more at ease with the chemistry of the blood that comes from a greater balance between oxygen and carbon dioxide.

In Yoga there are many breathing techniques to achieve different objectives - the so called Pranayama techniques.  Where does one start, however?

First, begin by self-assessment, so you know what your starting point is.  This will allow you to monitor your progress and to notice the changes that occur day to day, practice to practice.

In any comfortable position, close your eyes and draw your attention to the breath.  It helps to have a loud ticking clock nearby to mark the passage of seconds.  If you don't have something like that and it is quiet enough, you can count your heartbeat or your pulse.  Or say in your mind: "one Mississippi, two Mississippi..."  Use the same system of counting every time for consistency.

Count the length of your inhale and the length of your exhale.  Take several breaths, and average it out.  If your breaths naturally lengthen as you relax, count the longest breaths you can comfortably make.  Add the two numbers together.  For example, you counted your inhale to be 6, and your exhale to be 8.  Add them up, and you get 14 - this will be your base number.  This number may be different every time you begin your practice!  Then divide the number in half (round down to the nearest even number) - in our case we get 7.  Subtract 1 or 2 from this - and you get 5 or 6.  Let's take 5 - it is easier to start with a smaller number.  This is your Asana breath ratio.

When your Asana practice is not too strenuous you should be able to do your movements with a 5/5 count using Ujjayi breath.  For example, if you are doing child to table pose - Chakravakasana, inhale into table pose for the count of 5, and exhale back into child's pose for the count of 5.  Once your Asana gets a bit more demanding, you just let your breath drop down to any comfortable ratio, but during breaks come back to your original Asana ratio.

After Asana practice, after you have rested and are ready for Pranayama, start with that Asana ratio for a few breaths and then slowly come back to your original breath - 6 on in inhale, 8 on the exhale.  Use a soft Ujjayi breath.  Notice your mind now and its contents.  Do not judge yourself for what you discover, simply observe what is present.  Then begin to build the exhale component.  Inhale for 6 (and I am just using these numbers as examples - everyone will have their own unique number to work with).  Add a count or two to every exhale, over time building the exhale to be about twice as long as the inhale.  It is very important not to strain or force the breath in any way.  Visualize letting go of everything that you don't need in life with every exhale - the stress, the tension, the stagnation, self-defeating thoughts and criticism.  With time you should be able to maintain your 6/12 ratio very comfortably.  Add them together - you get 18.  Great - you have just lengthened your base number breath by 4 counts! 

Get as close as you comfortably can to doubling the length of your exhale in five-ten minutes.  If your exhale does not lengthen to double, just take whatever you get and begin to build the inhale component.  Use Ujjayi breath to add a count or two to every inhale, until it lengthens considerably, hopefully also doubling in length over time, so that both inhale and exhale are now equal in length.  Visualize breathing in the strength and courage to take change the habits that are unproductive in your life.  Take in new vitality with every breath.  Take 5-10 minutes to work on this.  And at some point you will be able to inhale for 12 and exhale for 12 without any strain whatsoever.  Congratulations!  Add them together and you get 24.

Then rest, and take soft Ujjayi breaths, counting again the length of the inhale and exhale as they naturally happen for you.  What tends to happen is that your breath lengthens nicely overall - you may be able to inhale for 10 and exhale for 10 - easy and soft.  No effort. 

The more you practice like this, the longer your base number breath will become with time.  Within a week or two of daily practice your Asana breath may lengthen by a count or two, and your base number will grow.  This will give your Yoga practice a new vigor, you will feel more endurance in your Asana practice.  You will feel more courage and strength in your daily life to cultivate new, healthy habits - like daily Yoga practice.  You will find yourself letting go of unhealthy situations and relationships and gravitating towards activities and people that bring greater balance to your life.  Your mind will be much easier to quiet down for meditation.

Once that happens, you are ready for the hold.  As you are meditating, add a little hold at the end of your exhale, for a count or two, or three.  Become comfortable with it.  Then stop holding the breath at the end of the exhale and instead add a hold at the end of the inhale.  Hold for no more than half of inhale or exhale in the beginning.  For example, if your exhale has grown to 10, then you can slowly build your hold to 5, and then switch to inhale for 10, hold for 5, exhale for 10. 

During the hold, allow your mind to drop fully into the point of stillness.  Allow your mind to become absorbed into the stillness.  The more completely you surrender to that stillness, the more you are able to see the difference between the things you can and cannot change - equanimity will grow and you will be able to see the world from a more balanced perspective.  That is what meditation is for - it rewards you with ability to make good decisions. An ability to pause before reacting, to act wisely, guided by the higher mind.

After a few days/weeks practicing it this way, add a hold at the end of both - inhale for 10, hold for 5, exhale for 10, hold for 5.  Add them all together and you get 30!  That is a very nice long breath ratio that opens the door for you towards many other Pranayama techniques, as well as Bandhas.  Ask an experienced Yoga teacher for help with those.

Watch my video on mastering the breath on my Youtube channel - Pranayama video.  The video is mostly geared towards beginners or even people who are not established in the Yoga practice, but it has some good information for everyone.