From http://www.catholic.org/clife/prayers/prayers.php?section_id=31&name=Marian%20(Mary) - which I assume to be the official Catholic website. I could be wrong about that - anyone know?
Immaculate Conception of the Virgin Mary
Father, you prepared the Virgin Mary to be the worthy mother of your Son. You let her share beforehand in the salvation Christ would bring by his death, and kept her sinless from the first moment of her conception. Help us by her prayers to live in your presence without sin. We ask this through our Lord Jesus Christ, your Son, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.
OR
Father, the image of the Virgin is found in the Church. Mary had a faith that your Spirit prepared and a love that never knew sin, for you kept her sinless from the first moment of her conception. Trace in our actions the lines of her love, in our hearts her readiness of faith. Prepare once again a world for your Son who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.