A principal requirement is faith, not just sincerity of belief that God exists and can hear prayers. (Hebrews 11:6) Such faith is demonstrated in striving to live in harmony with God’s righteous principles set out in the Bible. In his Sermon on the Mount, Jesus Christ stressed this point: "Not everyone who calls me ‘Lord, Lord’ will enter the Kingdom of heaven, but only those who do what my Father in heaven wants them to do."—Matthew 7:21, Today’s English Version.
Here's an example of those whose prayers would not be heard, the Hebrew prophet Isaiah wrote: "Even though you make many prayers, I [Jehovah God] am not listening; with bloodshed your very hands have become filled." (Isaiah 1:15) So any who do not respect the sacredness of life cannot expect their prayers to be heard, no matter how often and how fervently they pray. (hope this helps)