Fasting
“When you fast, do not look gloomy like the hypocrites. They neglect their appearance, so that they may appear to others to be fasting. Amen, I say to you, they have received their reward. But when you fast, anoint your head and wash your face, so that you may not appear to others to be fasting, except to your Father who is hidden. And your Father who sees what is hidden will repay you."
Matthew 6:16-18
Notice Jesus says "When you fast", not if you fast, but "When you fast". This means that Jesus expects you to fast. Just as the Prophets and Kings of the Old Covenant did, just as Jesus did, just as the Apostles and disciples did, just as the saints did and do. Fasting is essential, and fasting and prayer go together like bread and butter, rarely do you find one happening without the other. The reason for this is simple, fasting is a physical way to grow in your relationship with God.
Prayer occurs in the mind and fasting occurs in the body. Just as we are two part creatures: soul and body, our relationship with God should not neglect half of our being. Fasting, at its core, is the purposeful abstaining from the consumption of wanted food or drink. It is purposeful in that it is done for a purpose. We don't fast simply to fast, we fast to obtain graces, reparation for sin, to show obedience to God, to obtain graces or sanctification for other people, and to focus our prayer.
This is why Fasting is the other edge of our sword. Fasting is effective in our spiritual struggles. A man who is used to denying himself things he wants for the sake of himself, his family, and his friends, is better equipped to cut through temptation. Fighting through the thorn bushes of temptation with a dull blade will result in a battered and bruised knight at best and an ensnared knight at worst. Fast and pray to keep your mind and your body firmly in God's hands and ready to join battle with Satan when he attacks.