In recent years, there have been numerous books written on the Biblical practice of fasting. I have read at least one of those books. I have also read numerous chapters, articles, and blog posts about the practice. And I have heard many conversations about the practice of fasting.
And it seems to me that too many believers are confused about the Biblical practice of fasting — both about the command (or lack of command) to fast and the purpose of the practice. In my observation, many feel compelled to fast, though they may not be certain why they should; and many also experience guilt when they hear of the fasts of others or when it is revealed that they have never fasted.
As I discovered several years ago when I did a simple concordance search about fasting, there are some clear principles to guide the follower of Christ concerning fasting.
In summary, fasting was practiced in NT times, but never commanded — an important distinction, and the distinction which separates it from giving and praying in Mt. 6:1-18. When the NT writers talk about fasting, most often they are describing something that has taken place, or is taking place (e.g., Mt. 4:2; Lk. 2:37; Acts 13:2-3; 13:23; 27:9). Moreover, it is significant to note that the only two uses of the word group nesteuo after the book of Acts refer to “hunger” and not the practice of fasting. So the books that are prescriptive for the church and form the foundation of how the church should operate are entirely silent on the practice of fasting; after Acts, not only is fasting not proscribed, but it is not even referenced as an act practiced by members of the church. From this, it would seem safe to conclude that fasting is not a mandate for the church, though something which may be practiced (i.e., it is permissible to do and one would not be sinning in practicing fasting, but it is not something which must be done at any time by a believer).
For a more complete discussion of the NT teaching on fasting, download Fasting Summary.
Words of Grace 
Thank-you for this article; I too was a little confused about fasting. I have fasted for short intervals in the past, and feel that I have spiritually benefited from it, but I still felt guilt over not fasting longer and more frequently. I am glad that there is this liberty, permit, and option. I do believe that it is a good experience when practiced appropriately and most of SAFELY. When we do it, I feel it should be done purely; for example, fasting because we need to drop a few pounds, then calling it a spiritual fast is bad motivation. I think if I fast for spiritual benefits, I need to approach it in prayer, pray throughout, and then complete the fast in prayer, always seeking to glorify God and His answers.