In my most recent candidacy lesson for the Oblates of the Precious Blood I found this gem from Fr. Gerald Fitzgerald on mental prayer:
"Look at the beautiful face of our Mother Mary and you will say a perfect prayer. How simple it is to pray if we pray meditatively, if we take time to think of what we are saying. St. Francis of Assisi would spend all night saying only: "My God and my All." I want you to learn to pray like that, according to Divine Counsel: "When you pray, pray not long prayers." Pray meditatively; don't be racing through prayers. If you ask God, He will help you to pray. Pray as one who paddles a canoe on a quiet lake in the twilight. [Imagine] the violet banks of hills set against the gold of the setting sun; the lake almost a mirror with here and there a little fish splashing and a little bird winging its way home. You dip your paddle just enough to keep the canoe moving. So in meditation, one does well to dip one's paddle just enough to keep one's soul moving toward God."