If you're living a lifestyle of sin

Isaiah 59:1-4 "Surely the arm of the LORD is not too short to save, nor his ear too dull to hear.  But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear. For your hands are stained with blood, your fingers with guilt. Your lips have spoken lies, and your tongue mutters wicked things. No one calls for justice; no one pleads his case with integrity. They rely on empty arguments and speak lies; they conceive trouble and give birth to evil."

Ps 66:18 "If I had cherished sin in my heart, the Lord would not have listened"

Jer 14:10-12 This is what the LORD says about this people: "They greatly love to wander; they do not restrain their feet. So the LORD does not accept them; he will now remember their wickedness and punish them for their sins." Then the LORD said to me, "Do not pray for the well-being of this people. Although they fast, I will not listen to their cry; though they offer burnt offerings and grain offerings, I will not accept them. Instead, I will destroy them with the sword, famine and plague."

1Sa 8:18 "You will cry out for relief from the king you have chosen, and the LORD will not answer you in that day."

Matt 6:5 "And when you pray, do not be like the hypocrites, for they love to pray standing in the synagogues and on the street corners to be seen by men."

What kind of a man does God welcome into His presence?

Ps 15
"who may dwell in your sanctuary? Who may live on your holy hill?
        He whose walk is blameless and who does what is righteous,
        who speaks the truth from his heart
        and has no slander on his tongue,
        who does his neighbor no wrong and casts no slur on his fellowman,
        who despises a vile man but honors those who fear the LORD,
        who keeps his oath even when it hurts,
        who lends his money without usury and
        does not accept a bribe against the innocent.

What kind of man does God esteem?

Isaiah 66:2
This is the one I esteem:
        he who is humble
        and contrite (broken, teachable) in spirit,
        and trembles at my word

Therefore what can you do to become such a man?

Acknowledge sin and change your ways. Seek holiness:

Isaiah 1:15-17 When you spread out your hands in prayer, I will hide my eyes from you; even if you offer many prayers, I will not listen. Your hands are full of blood; wash and make yourselves clean. Take your evil deeds out of my sight! Stop doing wrong, learn to do right!

1. Thus this first stage in becoming a man of prayer involves Character Development - Sanctification - Holiness
We see this also spoken of Jesus who was qualified to be our high priest because of his holiness.  "Such a high priest meets our need— one who is holy, blameless, pure, set apart from sinners, exalted above the heavens." Heb 7:26 Thus if we are to walk as He did and become priests (men of prayer) on behalf of others, we first need holiness.

2. Secondly to be a man of prayer one has to grow with respect to love for your fellow man.

3. After this comes issues of content and discernment both of the circumstances and discernment of God's will.