Developing Concentration (Samadhi-sambojjhanga)

Lord Buddha stated:

Monks, there is a mental icon [nimitta] for Concentration (Samādhi-nimitta) and an icon to control wandering mind (Abbyagga-nimitta). One should contemplate these two Nimittas often. Such contemplation is food for the Concentration Factor of Enlightenment (Samādhi-sambojjhanga) which has not yet arisen to arise and the Samādhi-sambojjhaïga that has already arisen to develop to completion (Samyutta-nikāya, 19/370/97).

The commentary explains ten practices for giving rise to Concentration (Samādhi-sambojjhaïga) or developing it to perfection and eliminating the Hindrances which dim the Wisdom Eye (Chakkhu) (Samyutta-nikāya Atthakatha, 232-233), as follows:

1. Making Internal and External Objects bright and pure: [as for Investigation of Virtue, Dhamma-vicaya-sambojjhaïga].
2. Balancing the Mental Faculties (Indriya): [as for Investigation of Virtue, Dhamma-vicaya-sambojjhanga].
3. Using Mental Icons [Nimitta] intelligently: Meditating wisely with Kasina-nimitta.
4. Developing the mind at the proper time: When the mind is depressed, lacking effort and energy, the meditator should train the mind away from sluggishness by using Investigation of Virtue (Dhamma-vicaya), Effort (Viriya) and Bliss (Pīti). In this way, the Concentration Factor of Enlightenment (Samādhi-sambojjhanga) will arise.
5. Controlling the mind at the proper time: The meditator should control a mind distracted due to excessive diligence by using Tranquility (Passaddhi), Concentration (Samādhi) and Equanimity (Upekkhā). In this way, the Concentration Factor of Enlightenment will arise.
6. Brightening the mind at the proper time: When the Meditator feels negative and has weak exertion, wisdom, and tranquility, he should arouse disgust for the Five Aggregates by contemplating the Eight Urgencies (Saïvegavatthu) of the Rebirth Cycle (Vatta). These are: In the Past (1) Birth, (2) Decay, (3) Disease, (4) Death, and (5) Suffering in Hell (Apāya); In the Present (6) Current suffering and (7) Constant searching for food; And, finally (8) Future suffering in the Rebirth Cycle. Next, one develops Faith by recalling the Virtues of the Triple Gem. In this way, the Concentration Factor of Enlightenment will arise.
7. Concentrating effortlessly at the proper time: When the mind is not depressed, not distracted, and not negative, but tranquil and progressing properly with the right meditation practice, the Meditator should not make any exertion to control, train or brighten the mind. This is like a driver who makes no changes when the vehicle is running smoothly. It is effortless concentration at the proper time. In this way, the Concentration Factor of Enlightenment will arise.
8. Avoiding persons without concentration: Shun unfocused daydreamers with distracted minds, those focused on work, and those who have not yet attained Access Concentration (Upacāra) or Attainment Concentration (Appanā). In this way, the Concentration Factor of Enlightenment will arise.
9. Associating with Concentrated Meditators: Socialize only with those who have attained Access (Upacāra) or Attainment (Appanā) Concentration. In this way, the Concentration Factor of Enlightenment will arise.
10. Constantly directing the mind to concentration: The meditator continually guides the mind towards the arising of concentration in all postures, including both standing and sitting. In this way, the Concentration Factor of Enlightenment will arise.

The Concentration Factor of Enlightenment or Samādhi-sambojjhanga arises due to these ten causes and is fully developed to perfection when one attains Arahant Mentality.