Yoga sutras of Patanjali translated by Yogi Kalinath - Chapter 1, Verse 34
Pada 1, Sutra 34
Pracchardana-vidharanabhyam va pranasya
Or by the controlled expulsion and retention of the breath.
Control the breath and you control the mind. The two are virtually inseparable in regards to accessing various states of consciousness. We are a psychosomatic unit and breath is the energetic link between the soma (or body) and the psyche (or mind). It is a well known fact that calmness can be induced by deep rhythmic steady inhalations and exhalations. Conversely mental disturbances (citta vrittis going out of control!) and despondency are often accompanied by shallow irregular rapid and erratic breathing.
It is also interesting to note that the breath, particularly the in-breath and out-breath, activates thoughts. If you close your eyes and turn within you will sense that thoughts are energized by the in-breath and contemplation of those thoughts facilitated by the out-breath in its normal cycle. However, when the breath is held in (after an in-breath) or held out (after an out-breath), thinking is suspended. Truth in essence is beyond thought and the space between the inhalation and the exhalation can provide us with a gateway to Truth.
So become aware of your mind contents and bit by bit become detached from them by silently observing your breath. Slow down your breathing and make it even and you will get control of your mind.