One of the greatest men of prayer in Biblical history was Daniel.  We can learn much about fervent, effectual prayer - as well as being a righteous "man" - from him.  He prayed faithfully three times daily, even when doing so meant a lions' den!  Daniel studied Scripture (e.g. the scrolls of Jeremiah) to align his prayers with the Lord's revealed will.  An interesting fact about the super-powerful prayer of Daniel (9:1-19) is 72% of it was confessional. In contrast, we spend about 72% of our prayers in making requests! 

Daniel knew he and his people had no one to blame but themselves for their shameful state as captives in Babylon.  He repeatedly acknowledged Israel at fault for turning from God and ignoring His many warnings of impending judgment if she didn't repent and turn from her wicked ways.  Her sins were worse because she knew better!  

Learn why Daniel's prayer received immediate results, though God's messenger was delayed by a spiritual battle in the heavenlies!  The amazing answer to Daniel's prayer was the most extensive prophetic prophecy in the Old Testament - one you must study: "The Great Seventy Weeks' Prophecy" (see this podcast for Daniel lessons 33 and 34 on that amazing prophecy).