A couple of thoughts on fasting, and then a recommendation.
The holy Scripture appoints no time under the New Testament to fast; but leaves it to Christians’ own free choice (Rom. xiv. 3; 1 Cor. vii. 5), to fast as occasions shall be offered to them (Matt. ix. 15;) as when a man becomes an humble, an earnest suitor to God for the pardon of some gross sin committed; or for the prevention of some sin to which a man feels himself by Satan solicited; or to obtain some special blessing which he wants; or to avert some judgment which a man fears, or is already fallen upon himself or others; or, lastly, to subdue his flesh to his spirit, that he may more cheerfully pour forth his soul to God by prayer. Upon these occasions a man may fast a day or longer, as his occasion requires, and the constitution of his body and other needful affairs will permit (Lev. xxiii. 32; Josh. vii. 6; Esth. iv. 16.) - Lewis Bayly, The Practice of Piety
Dost thou fast? Give me proof of it by thy works! Is it said by what kind of works? If thou seest a poor man, take pity on him! If thou seest in enemy, be reconciled to him! If thou seest a friend gaining honour, envy him not! If thou seest a handsome woman, pass her by! For let not the mouth only fast, but also the eye, and the ear, and the feet, and the hands, and all the members of our bodies. Let the hands fast, by being pure from rapine and avarice. Let the feet fast, by ceasing from running to the unlawful spectacles. Let the eyes fast, being taught never1158 to fix themselves rudely upon handsome countenances, or to busy themselves with strange beauties. For looking is the food of the eyes, but if this be such as is unlawful or forbidden, it mars the fast; and upsets the whole safety of the soul; but if it be lawful and safe, it adorns fasting. For it would be among things the most absurd to abstain from lawful food because of the fast, but with the eyes to touch even what is forbidden. - John Chrysostom, Homily III, Concerning the Statutes
Two books that have been very helpful to me concerning the discipline of fasting have been Don Whitney's book, Spiritual Disciplines for the Christian Life and Richard Foster's book, Celebration of Discipline. Both have chapters on fasting that are must-reads.