Cor Iesu, patiens et multae misericordiae, miserere nobis...

Today we celebrate the Feast of the Sacred Heart. The Catholic Encyclopedia has a good article on devotion to the Sacred Heart, though surpsrising fails to mention the importance of devotion to the Sacred Heart of Jesus in the fight against Jansenism. Fr John Hardon SJ wrote this short article on the doctrinal basis of the devotion, whilst probably the most important magisterial document in this regard is Pius XII's Haurietis Aquas. Amongst the wonderful passages from the later, I'm particularly taken by:

59. Hence His words, actions, commands, miracles, and especially those works which manifest more clearly His love for us—such as the divine institution of the Eucharist, His most bitter sufferings and death, the loving gift of His holy Mother to us, the founding of the Church for us, and finally, the sending of the Holy Spirit upon the Apostles and upon us—all these, We say, ought to be looked upon as proofs of His threefold love.
60. Likewise we ought to meditate most lovingly on the beating of His Sacred Heart by which He seemed, as it were, to measure the time of His sojourn on earth until that final moment when, as the Evangelists testify, "crying out with a loud voice 'It is finished.', and bowing His Head, He yielded up the ghost."[56] Then it was that His heart ceased to beat and His sensible love was interrupted until the time when, triumphing over death, He rose from the tomb.
61. But after His glorified body had been re-united to the soul of the divine Redeemer, conqueror of death, His most Sacred Heart never ceased, and never will cease, to beat with calm and imperturbable pulsations. Likewise, it will never cease to symbolize the threefold love with which He is bound to His heavenly Father and the entire human race, of which He has every claim to be the mystical Head.

Newman's prayer to the Sacred Heart can be found here, whilst the traditional Litany of the Sacred Heart attracts a partial indulgence.