I often pray this Psalm (50 or 51 depending on the translation). This is the Ronald Knox translation and I feel that it really opens up the meaning- particularly for those who struggle with sins of the flesh. This was King David's psalm of repentance after his great sin of the flesh. King David is significant to me personally because of his own issues with lust- I mean his issue with lust lead to him murdering someone. However - he's the only person in the Bible who is called "a man of God's own heart". He's the first person in the Old Testament to fully repent of his sins- so if there's hope for David there's hope for me and you and our battle for purity.
HAVE mercy on me, O God, as you are ever rich in mercy; in the abundance of your compassion, blot out the record of my misdeeds. Wash me clean, cleaner yet, from my guilt, purge me of my sin, the guilt which I freely acknowledge, the sin which is never lost to my sight. You only my sins have offended; it is your will I have disobeyed; your sentence was deserved, and still when you give award you have right on your side. For indeed, I was born in sin; guilt was with me already when my mother conceived me. But you are a lover of faithfulness, and now, deep in my heart, your wisdom has instructed me. Sprinkle me with a wand of hyssop, and I shall be clean; washed, I shall be whiter than snow; tidings send me of good news and rejoicing, and the body that lies in the dust shall thrill with pride. Turn your eyes away from my sins, blot out the record of my guilt;my God, bring a clean heart to birth within me; breathe new life, true life, into my being. Do not banish me from your presence, do not take your holy spirit away from me; give me back the comfort of your saving power, and strengthen me in generous resolve. So will I teach the wicked to follow your paths; sinners shall come back to your obedience. My God, my divine Deliverer, save me from the guilt of bloodshed! This tongue shall boast of your mercies; O Lord, you will open my lips, and my mouth shall tell of your praise. You have no mind for sacrifice, burnt-offerings, if I brought them, you would refuse; here, O God, is my sacrifice, a broken spirit; a heart that is humbled and contrite you, O God, will never disdain. Lord, in your great love send prosperity to Sion, so that the walls of Jerusalem may rise again.Then indeed you will take pleasure in solemn sacrifice, in gift and burnt-offering; then indeed bullocks will be laid upon your altar.