Sivananda was a Self-Realized sage and the founder of Divine Life Society, Rishikesh. [Wikipedia]
1. If you can remain as an Akhanda Brahmachari, an unbroken celibate, for a period of twelve years, you will realize God immediately without any further Sadhana. You will have achieved the goal of life. Mark the word 'Akhanda'.
2. In a narrow sense, Brahmacharya is celibacy. In a broad sense, it is absolute control of all the senses. The door of Nirvana (liberation) or perfection is complete Brahmacharya. Celibacy is to a Yogi what electricity is to an electric bulb. Without celibacy no spiritual progress is possible. It is a potent weapon and shield to wage war against the internal evil forces of lust, anger and greed. It serves as a gateway for the bliss beyond, and opens the door of liberation. It contributes perennial joy and uninterrupted bliss. It is the only key to open the Sushumna (the chief among astral tubes in the human body running inside the spinal column) and awaken the Kundalini (the primordial cosmic energy located in the individual).
3. People talk of celibacy; but practical men are rare, indeed. A life of continence is really beset with difficulties. It is easy to tame a tiger or a lion or an elephant. It is easy to play with a cobra. It is easy to walk over the fire. It is easy to uproot the Himalayas. It is easy to get victory in the battlefield. But, it is difficult to eradicate lust. Lack of spiritual Sadhana is the main cause for all sexual attractions. Mere theoretical abstention from sensuality will not bring you good results. You must mercilessly cut off all formalities in social life and lead a pious life. Leniency to internal lower tendencies will land you in the region of suffering. Excuse will not be of use in this respect. You must be sincere in your purpose for the sublime life of spirituality. Half-heartedness will leave you in your old state of misery.
4. DANGER OF REACTION
a.) You will have to be very careful of reaction. The senses that are put under restraint for some months, or for one or two years, become rebellious if you are not always vigilant and careful. They revolt and drag you out when opportunities arise. Some people, who observe celibacy for one or two years, become more passionate and waste the energy considerably in the end. Some people become incorrigible, moral wrecks also.
b.) You must not labour under the delusion that you have eradicated lust completely by adjusting the diet a bit, by practicing Pranayama, and by doing a little Japa, and that you have nothing more to do. Temptation may overcome you at any moment. Eternal vigilance and rigorous Sadhana are very essential.
c.) You may be able to stop copulation for months and years, but there should not be any sexual craving or attraction for the opposite sex. The state of mental celibacy must be kept up even amidst temptations and sickness. Then only you are safe. The senses begin to revolt during times of ailment and also when you come in contact with sense-objects.
d.) Even advanced aspirants who have made great progress in Yoga should be very careful. They should not mix freely with members of opposite gender. They should not foolishly imagine that they have become great adepts in Yoga. A great saint of repute had a downfall. He freely mixed with lady disciples and allowed them to shampoo his legs. As the sex-energy was not completely sublimated and turned into Ojas, as lust was lurking in a subtle form in his mind, he became a victim. He lost his reputation. The sexual desire was only suppressed in him and when a suitable opportunity came, it again assumed grave form. He had no strength or will-power to resist the temptation. Another great soul who was regarded by his disciples as an Avatara became a Yoga-bhrashta. He also freely mixed with ladies and committed serious crimes. He became a prey to lust. What a sad misfortune! Aspirants climb with great difficulty by the ladder of Yoga and they are irrecoverably lost for ever on account of their carelessness and spiritual pride.
e.) You cannot attain perfect celibacy by limited effort. Just as a machine-gun is necessary to kill a powerful enemy, so also, constant, rigorous and powerful Sadhana is necessary to annihilate this powerful enemy, lust. You must not be puffed up with pride for your little achievement in celibacy. If you are put to test, you will hopelessly fail. You must be ever conscious of your shortcomings and you must constantly strive to get rid of them. The highest effort is necessary. Then you will have sanguine success in this direction.
5. A true Brahmachari in thought, word and deed has wonderful thought-power. He can move the world.
a.) If you develop strict celibacy, Vichara Sakti and Dharana Sakti will develop. Vichara Sakti is the power of enquiry. Dharana Sakti is the power of grasping and holding the Truth.
b.) If a man persistently refuses to yield to his lower nature and remains a strict celibate, the sexual energy is deflected upwards to the brain and is stored up as Ojas Sakti. Thereby the power of the intellect is intensified to a remarkable degree.
c.) The intellect becomes sharp and clear by continence. Continence increases infinitely the power of retentive memory. The strict celibate has keen and acute memory even in old age.
d.) A man who has the power of Brahmacharya can turn out immense mental, physical and intellectual work. He has a magnetic aura around his face. He can influence people by speaking just a few words or even by his very presence. He can control anger and move the whole world. Look at Mahatma Gandhi! He had acquired this power by constant and careful practice of Ahimsa, Satyam and Brahmacharya: non-violence, truth and celibacy. He influenced the world through this power alone.
e.) Through Brahmacharya and Brahmacharya alone can you get physical, mental and spiritual advancement in life. It is worth repeating that a true Brahmachari possesses tremendous energy, a clear brain, gigantic willpower, bold understanding, retentive memory and good Vichara Sakti.
f.) Swami Dayananda stopped the carriage of a Maharaja. He broke the sword with his hands. This was due to his power of Brahmacharya.
g.) Jesus, Sankara, Jnana Deva and Samarth Ramdas  were all Brahmacharins.
h.) It is worth repeating that a true Brahmachari possesses tremendous energy, a clear brain, gigantic will-power, bold understanding, retentive memory and good Vichara Sakti.
6. Practice of Brahmacharya gives good health, inner strength, peace of mind and long life. It invigorates the mind and the nerves. It helps to conserve physical and mental energy. It augments memory, will force, and brain power. It bestows tremendous strength, vigour and vitality. Strength and fortitude are obtained.The eye is the window of the mind. If the mind is pure and calm, the eye also is calm and steady. He who is established in Brahmacharya will have lustrous eyes, a sweet voice and a beautiful complexion.
a.) Practice of Brahmacharya by Sivananda Saraswati selfdefinition.org
b.) Divine Life Society HQ: website
c.) Importance of Brahmacharya by Sri Swami Sivananda at yoga-age.com