St. John of Kronstadt, On Prayer

In making the sign of the cross, believe and constantly remember that your sins are nailed to the cross. When you fall into sin, immediately judge yourself sincerely, and make the sign of the cross over yourself, saying: "Lord, Thou Who nailest our sins to the cross, nail also my present sin to Thy cross, and 'have mercy upon me after Thy great goodness;" (Psalm LI.3) and you wll be cleansed from your sin. Amen. 

St. John of Kronstadt, On Prayer