There is a critical balance to this spiritual exercise that requires constant vigilance. Not that you have to ride a line, but you do have to tip in the right direction. Simple deprivation is not enough to achieve the purposes of fasting; there has to be time set aside to read, pray, and listen as well. This is tricky. I'm the mom of a toddler who thinks she can't use the potty unless I'm sitting on the bathtub smiling at her. I get up with her in the morning, and before I know it, she's back in bed, my husband is brushing his teeth, and I'm dragging in the saddle thinking, "Wait, I meant to meditate today . . . as well as exercise, write a page or two, and make granola. Where did all my time go?"
The first step, nevertheless, is to give up something necessary to you. After that, other things will fall into place. All of my sources insist that true fasting consists of giving up food. They insist on it. Well, food is kind of necessary. It certainly drives the body's longings. And John Calvin notes, “We certainly experience that after a full meal, the mind does not so rise toward God as to be borne along by an earnest and fervent longing for prayer, and perseverance in prayer.” But I think that fasting should also mean giving up something that connects to the mind.
The other memory is tied to giving up processed sugar for 40 days. It was during this fast that I realized that the body does not require full mental consent to act. If the habit or craving is strong enough, the subject can be halfway through the action before she realizes what's happened. There was one morning when I found Fruit Loops in my cereal bowl and didn't remember pouring them. Then came the dilemma -- I had already poured milk on them. Should I eat them anyway or throw them away? You know sometimes God does require us to throw away an investment in the face of temptation.
So I would recommend giving up something physical and something mental at the same time. Look for something that you use when you're stressed or sad, an emotional crutch, or something that kind of dominates your diet mentally or physically.
Regarding food, there are a whole bunch of ways you can go about it. A lot of cultures give up the one or another of the animal proteins for a period. This is a traditional Lenten fast. Another traditional fast is to give up everything except bread, water, juice, honey, and nuts. Most of the Biblical fasts I have read about involve giving up food completely for a time. The children of Israel were instructed to do this on the Day of Atonment, and Daniel, Nehemiah, David, Esther, and Samuel all fasted this way at one time or another to seek help from God. Not everyone can do this. If you do fast completely, make sure you drink plenty of water and keep the fast to a reasonable length.
Then set aside time to pray and keep that appointment. Get others to help you keep it because everything and anything will want to interfere.
On the other hand, there is one fast where you may be inclined to give up everything. This is the spontaneous fast that comes from deep emotion or happens under the hand of the Holy Spirit. This is the kind of fast that Nehemiah undertook when he heard about the state of Jerusalem or Moses undertook on Sinai while waiting for the law. It's a fast that comes from deep distress that just takes away your appetite. You can honestly say, "I would rather pray than eat at a time like this." By praying at times like these, some people have averted catastrophes. Calvin puts such fasts under the discipline of the church and calls it a duty of pastors to sometimes call for congregational prayer and fasting when the Lord's judgment seems to be impending on a nation (Institutes 4.12.17).
Just be sure to be done when
you're done praying. You can't force the Lord's hand by giving up food
entirely. Fast. Pray. Put it in the Lord's hand. Be confident that you have been
heard. Then go eat.
Calvin, John. (1539). The Institutes of the Christian Religion, kindle ed. Henry Beveridge, Esq., trans. MobileReference.
Marilyn Hickey. The Power of Prayer and Fasting. The Christian Broadcasting Network. (Accessed February 16, 2012.) http://www.cbn.com/spirituallife/prayerandcounseling/intercession/Hickey_PrayerFasting06a.
Calvin, John. (1539). The Institutes of the Christian Religion, kindle ed. Henry Beveridge, Esq., trans. MobileReference.
Marilyn Hickey. The Power of Prayer and Fasting. The Christian Broadcasting Network. (Accessed February 16, 2012.) http://www.cbn.com/spirituallife/prayerandcounseling/intercession/Hickey_PrayerFasting06a.