"Descend with your attention into the heart, stand there before the Lord and admit nothing sinful to enter there. In this is the entire activity of inner warfare."
- St. Theophan the Recluse
Many of the parables in the canonical New Testament are about opening or healing the nous (many of the others are about access, retention, and use of spiritual energy, but that's for another post).
There are many methods for developing the noetic faculties, but the most important ones fall into two categories: Nepsis and Remembrance of God.
Nepsis, literally "watching", is the practice of self-observation. The Gospels frequently refer to the idea of sleep and the necessity for watchfulness. Metanoia, usually translated as repentance, is the change of consciousness that occurs after many years of watchfulness, in which we engage in exercises in self-observation that develop the ability to discern between hylic, psychic, and pneumatic influences, and thus helps us "tune in" the noetic faculties to the pneumatic center.
Remembrance of God is a practice found in traditional schools of spirituality worldwide. Many Muslims practice dhikr (literally "remembrance" or "invocation"), following the instructions of the Holy Quran: "Those who believe, and whose hearts find satisfaction in the remembrance
of Allah: for without doubt in the remembrance of Allah do hearts find
satisfaction." Hindus and Buddhists have developed a rich tradition of japa, praying with a mantra. Many don't know esoteric Christianity also has a highly developed tradition of practicing the remembrance of God.
One method is found within the Eastern Christian practices known as the hesychast tradition, centering on the practice of the Jesus Prayer. This short prayer, "Lord Jesus Christ, Son of God, have mercy on me, a sinner," is repeated in on woolen prayer rope. The best way to begin with this kind of prayer is to pick up a copy of "The Way of A Pilgrim" and a prayer rope and begin with a manageable number of recitations - 100 is a good start.
Another method of Remembrance is the invocation of IAO, a Gnostic name of the Ineffable Father. Fr. Tony and I did a Talk Gnosis episode about using the name a few months ago - you can check it out here.
If we think of the Nous as a mirror, the use of the Divine Name, whether by saying the Jesus Prayer, the IAO chant, or other method, acts to polish and clean away the grime, so that it can accurately and clearly reflect the light of the Spirit in the Pneumatic center.