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.”

This entire passage has Jesus Himself giving very practical advice on how to pray… praise God, ask for His will to be done before yours, make your needs known to Him, repent and forgive others.  And know that while you are fasting, He sees what is done in secret, and will reward you.