Fasting is one of those tools that is so basic it can almost be said to do "everything." On a spiritual and psychological level, fasting promotes clarity, purification, a ritual taking-of-control in one's life, devotion to a deity, principle, or saint, a method of asserting the power of the spirit over the flesh, and much, much more. It can be used to create a sacred space, to celebrate certain holiday's of spiritual significance, or to connect yourself to the particular practices of your tradition. Fasting can also be used to intensify the effects of other practices, such as meditation or prayer.
On a physiological level, a fast can be very healthy, particularly for cleaning toxins out of the body. Toxins are typically stored in body fat, and as body fat is burned of in a fast, toxins are also removed. There is much medical research to also support the health benefits of caloric restriction generally.
Fasting can be used in a political way as a method of protest. Famously, Mahatma Gandhi used fasting to protest British policy in India, and thereby helped to bring about freedom for hundreds of millions of people.
Restrict some or all food and/or drink for a defined period of time.
- Decide which form of fasting you would like to do. There are literally thousands of fasts that have a pre-defined ritual observance. Maybe you would like to do one of these. On the other hand, you can simply go with your own intuition.
- Decide upon how much fasting time is right for you. A fast can be as short as a few hours, or as long as a week. Some forms of semi-fasting, in which some food intake is permitted, could go on for much longer.
- Set aside this time as a special, sacred holiday in which you will be observing your fast. Make whatever arrangements are necessary to support your fast.
- Begin the fast with an appropriate ritual. In the case of a traditional fast, there will be traditional rituals for beginning. Minimally, take a moment to clearly state your purpose for doing the fast. Get clear about your intention.
- During your fast, monitor very carefully your physical condition. It is important to not overdo it. If you begin feeling faint and experiencing an overwhelming desire for food, it is probably best to eat.
- When your fast is complete, do whatever rituals are traditional for the breaking of fast. Minimally, remember your reasons for doing the fast, and reflect on how this “time out of time” has been for you. Return to eating in a careful manner.
Fasting is specifically mentioned in the Bible, the Torah, the Koran, the Mahabharata, the Upanishads, and many other sacred source texts of the world's religions. It is certainly one of the oldest, most basic, and most well known of all spiritual practices. Here is a huge wikipedia page on fasting, which touches on most of the religious references.
Fasting comes with some important precautions.
First and foremost, people can die from not eating enough food. It is extremely important that fasting be done with tremendous caution. Do not overdo it!
Secondly, restricting or completely stopping your caloric intake can drastically change your psychology. It is probably best to do fasting when you are not working, and during a time (such as a weekend) when you can excuse yourself from all responsibilities. You may make decisions or take actions that you normally would not.
Thirdly, some people manifest psychological illness or mental difficulties in the form of fasting. Anorexia, for example, is a pathological form of fasting. If fasting takes on a compulsive or unstoppable aspect for you, it is important to seek out a psychological professional for help.
The internet contains literally tens of thousands of pages about fasting of various sorts. If you are interested in a specific fast, there is likely to be information available about exactly how to do it, as well as support groups, other practitioners, etc., available.
A huge fasting website, with lots of resources
A medical article about fasting - how to - written by an M.D.
Campus Crusade for Christ web page on fasting
Simply doing a Google search on "fasting" will reveal a universe of fasting options!