Paruresis is a type of phobia in which the sufferer is unable to urinate in the (real or imaginary) presence of others, such as in a public restroom.

A proven method of physically treating paruresis is through breath holding. The paruretic person takes a deep breath in, exhales about three-quarters of it, and then holds their breath for a given length of time, usually 60 seconds or less, and in doing so, the internal sphincter muscle of the urethra relaxes enough so that the person can urinate freely. http://en.wikipedia.org/wiki/Paruresis#Treatment