If you look back into your Bibles there are many desperate outreaches from the men of God whom declare a fast and cry out to God for help. Typically in the Old Testament, it's when they feel backed into a corner and are desperate for God's help or they will perish. The examples in the New Testament are of a time to draw closer to God or to seek direction.
Usually a fast is done for a specific reason.
Pick what you want to bring before God. Then when you are feeling the pains of hunger go into a time of prayer and bring your needs before God. This is the best way to draw closer to God.
Remember as our Heavenly Father, He wants to bless us... even more than we can believe!
It is written,"You have not, because you ask not."
Do you not think that God can do all things?
Do you not believe that God is the same today as he was back in the Old Testament days?
Do you not think that if one of His children cry out for help, He will help them?
Do you doubt Him?
Doubt is not of God...therefor it must be understood to be the seed of the devil. So, do not doubt God! He is good all the time! Believe that, get it into your heart and soul!
And when you are in a desperate situation, come before God in prayer and fasting and believe that He wants to help you.
It is also written that it glorifies God to answer. The only conditions given to this is that you must believe in your heart and mind and speak the words of faith, as if you are getting it...even before you see it.
Remember even Daniel had to wait 21 days before the angel appeared to him and said that God had already answered, but there was a war waging in the spirit realm for the whole 21 days!
So do not give up, and do not let go of God! He will never let go of you!