Jesus promised that He would give His disciples the command and privilege to fast after He ascended. Fasting A.D. is powerful because the Lord has decided to make it the only way He drives out certain demons (Mt 17:21, NAB). As Jesus fasted forty days and defeated Satan, so do we (see Mt 4:2ff).
Because fasting has an irreplaceable part in New Testament spiritual warfare, we should be very careful when we fast. First, we must be totally committed to the Lord Jesus. Only under His authority are we in authority over the devil. Next, we must be repentant of sin and be forgiving others' sins. Otherwise, we cut ourselves off from God and are sure to be defeated. In New Testament fasting, we should usually fast communally. One-man armies don't win wars. We need each other in the body of Christ (1 Cor 12:21ff). In A.D. fasting, we must have our houses in order (see Is 38:1). If there's anything wrong with our families, marriages, relationships, or lifestyles, Satan will viciously exploit these weaknesses. In effect, we must carefully obey the Lord, for when our obedience is complete, we will bring down Satan's strongholds (2 Cor 10:4-6) and quickly crush Him under our feet (Rm 16:20). Fast in Jesus. Clear out demons.