Fasting

The focus of fasting should not be on the lack of food.
Fasting from food can be done for a variety of purposes, either physical or spiritual. So abstaining from food alone doesn't constitute a Christian fast. Instead, a Christian fast is accompanied by a special focus on prayer during the fast, often substituting the time you'd spend eating with prayer.

Fasting provides a real-life illustration of dependency.
Although modern man thrives on the idea of being independent, beholden to no one, fasting helps you put the facts in the proper perspective. It's easy to believe in your independence with a full stomach, but when you start to feel hunger pains in your belly after missing a meal or two, you awaken to your body's dependency on food to survive. Fasting reveals a physical reliance on food that points to the ultimate dependency – the fact that you're dependent on God for things far more important than food.

Fasting fosters concentration on God and his will.
Oswald Chambers once said that fasting means "concentration," because when you're fasting, you have a heightened sense of attentiveness. Food or any physical sensation can satisfy, fill you up, and dull your senses and spiritual ears. In contrast, a hungry stomach makes you more aware and alert to what God is trying to say to you.

Fasting offers a way to impose self-control in your life.
It gives you a "splash in the face" to awaken you to the need for the personal strength of will that you need to grow spiritually. When you restrain yourself physically, you'll find it easier to apply this same self-discipline in your spiritual life.

One last thing – everyone can participate. Not everyone may be able to fast from food (pregnant women and diabetics for example), but everyone can give up something in order to focus on God (e.g. unplugging the television for 24 hours could also be an effective way of joining the fast)!

Source: Richard Wagner, Christianity for Dummies.