If I regard iniquity in my heart, the LORD will not hear me.
Prayer at its best is the expression of the total life, for all things else being equal, our prayers are only as powerful as our lives.
In the long pull, we pray only as well as we live!
Some prayers are like a fire escape, used only in times of critical emergency—never very enjoyable, but used as a way of terrified escape from disaster. They do not represent the regular life of the one who offers them; rather, they are the unusual and uncommon acts of the spiritual amateur.
Most of us in moments of stress have wished that we had lived so that prayer would not be so unnatural to us and have regretted that we had not cultivated prayer to the point where it would be as easy and as natural as breathing.
No Christian wants to live his whole life on an emergency level. As we go on into God we shall see excellency of the life of constant communion where all thoughts and acts are prayers, and the entire life becomes one holy sacrifice of prayer and worship!
To pray effectively it is required of us that there be no unblessed areas in our lives, no parts of the mind or soul that are not inhabited by the Spirit, no impure desires allowed to live within us, no disparity between our prayers and our conduct.
Undoubtedly the redemption in Christ Jesus has sufficient moral power to enable us to live in a state of purity and love, where our whole life will be a prayer!
66:18 If I had considered evil in my heart The psalmist asserts that if he had been living an unrepentant, sinful life—or desired sin—then God would not have heard him (Prov 28:9). However, because he was innocent, God heard his cries and delivered him (see Psa 17:3–7).
66:18 “Regard” means here “to look with favor upon” or “plan.” The believer must take the same attitude toward sin which God takes. A clear conscience, i.e., one devoid of a desire for sin, is necessary for effectual prayer (cf. Is. 59:2; Heb. 10:22; 1 John 3:21).
 Tozer, A. W., & Smith, G. B. (2015). Evenings with tozer: daily devotional readings. Chicago, IL: Moody Publishers.
 Barry, J. D., Mangum, D., Brown, D. R., Heiser, M. S., Custis, M., Ritzema, E., … Bomar, D. (2012, 2016). Faithlife Study Bible (Ps 66:18). Bellingham, WA: Lexham Press.
 Criswell, W. A., Patterson, P., Clendenen, E. R., Akin, D. L., Chamberlin, M., Patterson, D. K., & Pogue, J. (Eds.). (1991). Believer’s Study Bible (electronic ed., Ps 66:18). Nashville: Thomas Nelson.