Anna Hazare seems to have studied Gandhi's fasting techniques closely

Water and willpower kept fasting Mahatma in form

Having never kept a fast except when I was getting married (some strange custom of Paschim Banga!), I have failed to understand how people can go without food.

But this is the season of fasting (of the Gandhian kind - a technique that Anna Hazare seems to have mastered), so it may be illuminating to find out how people survive long fasts like the Mahatma, who was the champion in the department, unlike Baba Ramdev, who made a spectacle of himself. It's always interesting to see from foreign eyes an event as headline grabbing as Gandhi's last fast against the Raj in 1943. Read the report Time magazine ran of the fast and you'll see how the western world viewed the fast as a superhuman effort. "Midway through his ordeal the act of drinking water exhausted him," Time magazine reported on the 73-year-old Mahatma's 21-day fast. "A panel of nine doctors announced that Gandhi's 'uremic condition deepens and if his fast is not ended without delay it may be too late to save his life'. He was too far gone for blood transfusions or glucose injections to be of help.

Government bulletins prepared Indians for news of his death. Only a body and a will that have survived a lifetime of fasts and jailings kept Gandhi alive."

Fasting came easy to Gandhi. He went on protest or 'purification' fasts 17 times in his life, say the ultimate trivia collectors, David Wallechinsky and Irving Wallace in The People's Almanac. The tradition started with his first famous fast from March 15, 1918, which made Ahmedabad mill owners, who'd declared a lockout, rush to the negotiating table and seal a settlement with the striking workers whom Gandhi led. It was over by March 18 - a short and successful fast, which, ironically, was provoked by the locked-out mill workers reproaching Gandhi for not understanding how it was like to go hungry.
Gandhi had grown up seeing his mother keeping long and exacting ritual fasts, and his own attitude towards food must have conditioned him for protest fasts, which he saw as the "truest prayer" that "crucified the flesh" and to that extent "liberated the soul". The Mahatma had once said about food, "Experience has taught me that it was wrong to have dwelt upon the relish of food. One should eat not in order to please the palate, but to keep the body going." Many have used fasting as a political weapon, from Bobby Sands, Provisional Irish Republican Army volunteer and MP who died in prison on the 66th day of his protest fast in 1981, to prisoners at Guantanamo Bay, but few in human history mastered the art of keeping "the body going" as well as Gandhi. Water, "with or without salts and sour limes" (as he pointed out in an article in his newspaper, Harijan, on January 18, 1948), was the only luxury he allowed himself when kept protest fasts.
The Mahatma had perfected a routine for long fasts. Before going on one, he would have lemon juice and honey with warm water. He would keep having water, occasionally with salt or lemon juice, through the day, no matter how nauseous or weak he felt. To minimise the loss of energy, he would sleep more than usual. And as Time magazine reported in 1943, "Each morning the Mahatma was wheeled on his bed to a palace bathroom to be shaved and washed. He was massaged twice daily, had mud packs placed on his head. He was given occasional enemas." (The "palace" was Pune's Aga Khan Palace, where he was under detention.) The custodians of medical wisdom will advice you not to keep a fast with only water as your nutrient.
Have plenty of fresh fruit and raw vegetable juices, they say, because they're easy to digest and have all the nutrients you need. Gandhi, though, followed his own wisdom and it seemed to work.