Concentration or Dharana is centering the mind on one single thougt.
When there is deep concentration, there is no consiousness of the body and surroundings.
When the outgoing tendencies of the mind are arrested, when the mind is restrained within the heart, when all its attention is turned on itself alone, that condition is Antarmukha-Vritti (inward-moving mind).
When you are firmly established in the idea that the world is unreal, Vikshepa (through names and forms) and Sphurana of Sankalpas (thoughts) will slowly vanish.
Do not wrestle with the mind during concentration.
Keeping the balance of mind in pleasure and in pain, in heat and in cold, in gain and in loss, in success and in failure, in praise and in censure, in respect and in disrespect, is real wisdom.
No work can be done successfully without calmness and concentration.
A continuous flow of perception or thought is Dhyana (meditation).
The food for the soul is more essential than the food for the body. Therefore do your prayers, Japa and meditation regularly.
Through introspection you will have to water this garden of mind and with meditation and sublime thinking remove the weeds of vain, useless, discordant thoughts.
Process of Meditation:
Keeping down the senses (Pranas), subduing his desires, and gently respiring through the nostrils, let the wise diligently attend to the mind, as the charioteer to a car, drawn by vicious horses.
Concentrate and meditate on the expansive sky.
Concentrate on the Anahata sounds of the heart by closing the ears.
Concentrate on the breath with Soham repetition."SO" during inhalation and "HAM" during exhalation. SOHAM means "I AM HE".
Concentrate on the blue sky.
Concentrate on the all-pervading of the sun.
Concentrate on the various Chakras of the body.
Concentrate on the abstract ideas of Satyam, Jnanam, Anantam, Ekam Nityam, etc. (Truth, Wisdom, Infinity, One Eternal Essence). Lastly stick to one thing only.
When the mind becomes Nirvishaya (free from thinking of sense-objects and their enjoyments) it is meditation.
Whatever spriritual practice you do, either Japa, practice of Asana, concrete meditation on Saguna-murti or Pranayama, do it systematically and regularly every day
A mixture of Tandri (drowsiness) and Manorajya (building castles in the air, reverie) is mistaken by aspirants for deep meditation and Samadhi.
Sometimes a big sun or moon or lightning-like flashes appear in front of the forhead during meditation. Do not mind these appearances.
Sometimes Devatas, Rishis, Nitya Siddhas will appear in meditation. They appear before you to help and give you encouragement.
Repeat the name of the Lord or pure OM. Think of His atttributes, Omniscience, Omnipotence, Omnipresence, etc. Your mind will be filled with purity.
Obstacles in Meditation:
Fear of death, fear of desease, sense-desire, flurry and worry, sloth and torpor, greed, should be removed.
