Learning how to breathe is key to having a fun run and is necessasary to help us avoid the dreaded side ache. 

When I run I used a breathing pattern that corresponds with my foot strike, thus allowing my number of breaths to increase as I move faster or decrease as I slow down.

It is a 4 count pattern that is as follows:

Step 1:  Exhale
Step 2:  Exhale
Steps 3 & 4:  Deep Inhale

At first you may have to exaggerate your breathing pattern in order to teach your body how to breath when you are exercising.  Eventually your body will adapt your breathing automatically. 

Be careful not to over-exaggerate your breathing or you may hyperventilate.