Pope Shenouda III
  Prayer is opening the heart to God, so He would enter  and purify it.
  It reminds us  of the prayer of the tax  collector who lifted up a contrite heart  before God, asking for His  mercy (Lk 18:13). And so he went back to his  house justified. Therefore, you  should pray to acquire purity of the  heart, while you say to God in your prayer: “Purge me with hyssop, and I shall be clean,  wash me, and I shall be whiter than snow” (Ps 50)... Is not He the  one who said, “I  will give you a new heart and put a new spirit  within you... I will  put My Spirit within you and cause you to walk in  My statutes” (Ezek 36:26-27).. Ask Him in your prayers to fulfil this promise.