Thomas Boston, Works 2:560:
See the absolute necessity of prayer in the Christian life. Be convinced, that ye cannot be without that which the Lord is at so much pains to bring you to, by not only teaching you what to pray, but promising his Holy Spirit to assist you therein. Apply yourselves diligently to this duty, that you may be often with God, guiding yourselves therein by the direction of the word; and plead importunately for the quickening power and influence of the Holy Spirit, for his help and assistance. He will show thee thy wants, to give thee matter of petition; thy sins, to give thee matter of confession; the mercies and blessings of God, to yield thee matter of thanksgiving; and the church’s miseries and necessities, to furnish thee with matter of intercession.
Source: http://www.puritanboard.com/f25/see-absolute-necessity-prayer-83280/, Comment #1 (Thank You, Reverend Winzer!)