"Whenever you fast, do not put on a gloomy face as the hypocrites do, for they neglect their appearance so that they will be noticed by men when they are fasting. Truly I say to you, they have their reward in full. But you, when you fast, anoint your head and wash your face so that your fasting will not be noticed by men, but by your Father who is in secret; and your Father who sees what is done in secret will reward you." -Matthew 6:16-18
Though fasting is not widely practiced in affluent American churches, it is a spiritual discipline in which the Christian should regularly be engaged. A quick survey of Scripture will show that such things as times of trouble, urgent desires, and missionary expansion are all perfect opportunities to meet with prayer and fasting. The practice of deliberately withholding from your body normal foods, drinks and pleasures, known as fasting, done correctly, opens up an avenue to the Father's heart.
I emphasize "done correctly." I speak not so much of how often or how long or what type of fast, but the audience you seek. Jesus' words above tells us that if the attitude of our heart when we fast is to be seen by others, then we will get a reward. What is it? Well, simply that we get what we want. Others will see us, i.e., our hunger to be noticed will be satisfied when we seek the attention of men. Men will notice us, but is that really what the godly should yearn for?
No, when we fast let our hunger drive us to seek the attention of the Father. With joy on our faces and anticipation in our hearts, let every hunger pain or unmet desire be redirected toward getting the attention of the Father through prayer. For if our heart's hunger is to be noticed by God, what does Jesus say will be the result? Again, we get what we want. He will notice and will reward us.
So if you are in a trial, have gone too long with an unfilled desire, or want to see others reached for the gospel, give some time to fasting and prayer. Those who take a few trips on "the fast lane" are never disappointed in the end, for they ultimately arrive into the very presence of the Father Himself. And when we are there, even if the answer to our prayers does not come in the manner or timeliness we want, our every desire will be fulfilled in Him.
If you want a wonderful, comprehensive treatment of this subject, read John Piper's Hunger for God.