If one turns away his ear from hearing the law,
even his prayer is an abomination.
God is disgusted with those who come to Him in prayer while ignoring Him in His Word. He finds such prayer to be an abomination. The word "abomination" simply means repellent or disgusting. The surest way to unanswered prayer then is to disregard scripture (God's law) and then go to God expecting an answer to prayer.
This is often why prayers are hindered. I know of at least one clear warning in the New Testament that echoes the sentiment of this proverb. It is written to husbands in 1 Peter 3:7 and I consider personally to be the scariest motivator in the Bible. It warns men to love and understand our wives, honoring them lest our prayers be hindered. So to disobey God's Word by manipulating my marriage to get what I want is to render my spiritual walk disgusting to God as well.
God's truth matters.
It seems that effective prayer is linked to obedience to scripture as God's Word leads to change within my heart through repentance, belief, and obedience. Disgusting prayer sees God as nothing more than the wish-granter and little else. It makes me the authority and God the servant, rather than the other way around. Effective prayer is part of my understanding of the Lordship of Christ ordering all things for the glory of God. Jesus as Lord leads both my prayers and my obedience.