[Read] 16 “When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show others they are fasting. Truly I tell you, they have received their reward in full. 17 But when you fast, put oil on your head and wash your face, 18 so that it will not be obvious to others that you are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret, will reward you. (Matthew 6:16-18)
[Meditate] "Whom am I trying to please by my religious practices? Honest reflection on that question can produce most disquieting results." -D.A. Carson, Jesus' Sermon on the Mount, 79.
The Pharisees fasted twice a week (Luke 18:12), on Mondays and Thursdays. Multiple people suggest the Pharisees are who Jesus had in mind here, and they might very well be correct... I just haven't found a reference for this, so I'll consider it conjecture at this point.
[Pray] God, you care about my heart. You desire not that I do religious things such as fasting to impress others. If and when I fast, my heart needs to be one bent on pleasing you. Help make me aware of my motives through your Holy Spirit. Convict me of when I go astray in this. Give me a heart to please you instead of using you for selfish motives. In Jesus name, Amen.
[Live] Don't draw attention to myself when I fast. Don't use spiritual things to impress people.