So, Jesus said to them, “Because of your [e]unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you. However, this kind does not go out except by prayer and fasting.” (Matthew 17: 20-21)
The best advise I ever got about a relationship with God is that it is a marathon not a sprint. Much the same applies to fasting and praying. While seasons of fasting and prayer may seem like sprint, we should always have it in our foremost thoughts that it is a marathon. And so, the goal must be to make it a regular part of our spiritual lives – this typically means quality over quantity and regular cadence over Guinness book of world record efforts. We must also guard against any form of legalistic bondage. In Galatians 5:18 Paul says, “If you are led by the Spirit, you are not under the Law.” This means fasting and praying is not a one size fits all, it also means that we can adjust as we get feedback from the Holy Spirit and from our bodies.
Derek Prince in his book Shaping History Through Prayer and Fasting gives some insightful tips
Enter into fasting with positive faith. God requires faith of this kind in all who seek Him. “Now without faith it is impossible to please God.” (Hebrews 11:6)
Remember that “Faith comes by hearing and hearing by the word of God” (Romans 10:17) Fasting should be done based on conviction from the scriptures and nothing else
Fasting is not an emergency operation that we only do when faced with challenges. It is better to begin fasting when you are spiritually up. “God is the rewarder of those who earnestly and diligently seek Him." (Hebrews 11:6)
If you are new to fasting, begin with reasonable efforts not Olympic medal performances. If you set too lofty goals and fail to meet it you may become discourages
Fasting without prayer and bible study is ineffective. (In my view this is just self-inflicted starvation and nothing more). “But we will give ourselves continually to prayer, and to the ministry of the word.” (Acts 6:4)
Like new year resolutions, set goals for your fast. Write down what you want God to do through your fasting and prayer. "Do not be anxious about anything, but in everything by prayer and supplication with thanksgiving let your requests be made known to God." (Philippians 4:6)
Avoid religious ostentation and boastfulness. Fasting is an engagement between you and God, it is not a badge of righteousness. Apart from periods of prayer and other spiritual activity, your life should be as normal and unpretentious as possible. “Moreover, when you fast, do not be like the hypocrites, with a sad countenance. For they disfigure their faces that they may appear to men to be fasting. Assuredly, I say to you, they have their reward. But you, when you fast, anoint your head and wash your face, so that you do not appear to men to be fasting, but to your father who is in the secret place; and your Father who sees in secret will reward you openly.” (Matthew 6:16-18)
Each time you fast, check your motives and check them again and again. Understand the motives and attitudes that are pleasing to God and ones that are not