Here are several types of intentional breathing exercises. It is important to be aware of your breathing, and be able to switch in to abdominal breathing for all of them, so I suggest you become comfortable with "relaxing breath" first. While breathing exercises  can be learned quickly, it often takes months of practice to realize  the full benefits of them, so don't give up!
> Breathing exercises are 
> important for meditation magick and healing.
> Breath is used by many 
> mental health professionals as a healing
> technique for such 
> conditions as anxiety, panic attacks, phobias,
> depression and stress 
> symptoms such as tension headaches, tight
> muscles, irritability and 
> fatigue. It is also great for PMS! :)
> --Zaratyst
> 
> Basic Natural Breathing
> Sit, stand or lie comfortably with your back in
> a good posture. 
> Slumping forward inhibits your ability to
> breathe deeply and fully. 
> Identify anything that is stressing or worrying
> you. Only think 
> about this for a few seconds, then let it all
> go. Imagine the 
> tension taking flight like a flock of doves
> soaring into the sky. 
> Hear their wings flapping as your worries leave
> you, lighter, freer. 
> Consciously relax your arms and shoulders.
> Place one hand on your 
> abdomen. 
> Slowly exhale through your nose, expelling all
> the air you can 
> without strain. 
> Take a deep breath, filling first your abdomen,
> then your chest. 
> Feel your abdomen inflate like a soft balloon,
> but not distend. 
> Exhale slowly through your nose again. 
> Repeat this, being cognizant of each breath
> until your breathing is 
> steady and natural feeling. 
> Notice how relaxed you have become. You are
> ready to proceed with 
> grounding and centering, meditation, more
> advanced breathing 
> exercises (below) or other activities. 
> 
> 
> 
> ---------------------------------------------------------------------
> -----------
> 
> 
> Tension Release Breathing
> Do not let yourself think about any worries or
> anxiety provoking 
> issues while doing this exercise. If the
> thoughts creep in, stop 
> them by re-focusing on the breathing and
> counting. 
> Begin breathing deeply, filling your abdomen
> first, then your chest. 
> Sit quietly, scan your body for tension, and
> consciously tell any 
> tense muscles to relax (tension often
> accumulates in shoulders, neck 
> and back). 
> Inhale deeply through your nose, pause, then
> exhale, count one. 
> Repeat, counting your exhalations to five. 
> Begin again, counting exhalations one to five. 
> Continue this exercise 5-10 minutes. 
> You should be relaxed and the anxiety or
> tension greatly diminished.
> 
> 
> 
> ---------------------------------------------------------------------
> -----------
> 
> 
> Purifying Breath
> This breathing exercise is used to tone your
> respiratory system and 
> to send renewed energy to your whole body. If
> you have been sitting 
> and feeling stagnant, this one will reawaken
> you. It can be used as 
> part of a pre-ritual cleansing as well.
> 
> Sit or stand comfortably. 
> Inhale a complete, natural breath through your
> nose. 
> Hold the breath for a few seconds. 
> Pretend you have a straw in your mouth, and
> exhale a short burst of 
> air forcefully, through the small opening.
> Stop, then do it again. 
> With each puff out, visualize an pollutants,
> negativity and 
> detrimental germs going with the air, and
> falling harmlessly to the 
> floor. 
> Repeat until you have emptied your lungs via
> these short, strong 
> puffs. 
> Inhale, visualizing pure, cleansing air
> entering your body, filling 
> you with purity. 
> Repeat about 6-10 times. 
> 
> 
> ---------------------------------------------------------------------
> -----------
> 
> 
> Energizing Breath
> Stand comfortably. 
> Stretch several times, reaching higher each
> time. 
> Trigger your yawn reflex while stretching. 
> Begin breathing deeply and naturally, as above.
> 
> Spin your arms backward, all the way around. 
> Switch directions, spinning them forward. 
> This will re-energize you! 
> 
> 
> ---------------------------------------------------------------------
> -----------
> 
> 
> Healing Breath
> Lie comfortably. 
> Practice natural breathing for a few minutes. 
> Place your dominant hand on the part of the
> body that is injured or 
> infected. Place your other hand (receptive) on
> your abdomen. 
> Visualize energy flowing into you with each
> inhalation, filling a 
> reservoir in your solar plexus. Imagine this
> energy flowing into the 
> selected area, washing through it, driving out
> the pain or 
> infection. Cleansing, purifying, disinfecting
> energy being drawn 
> from the universe and being channeled into
> healing light. 
> Continue this until your feel ready to be
> finished. 
> Place both arms at your sides, palms turned
> downward and let the 
> excess energy in you hands flow back into the
> surface/ground beneath 
> you. 
> This exercise may be repeated as needed. 
> 
> 
> 
> 
> Text copyright © 1997, Zaratyst, it may be
> freely distributed, 
> please keep copyright attached.