The first prayer I remember saying as a child was "Jesus, Mary, Joseph: I give you my heart, my soul, Amen." It was a very good prayer for a very little kid, who had yet to be able to remember and recite all of the Our Father, Hail Mary, or Glory Be. To a little kid, the fact that the baby Jesus had a mommy and a daddy was reassuring; Jesus wasn't a ghost or a spirit. He was a real boy, who was also really God, a starting point into the Mystery of the Trinity for a four- or five-year-old.
The Church has a special name for very short prayers: they are called ejaculations. (Yes, there's a ten-year-old somewhere in me too who finds that term hilarious.) The link is full of nice quoted reminders that praising God all day, at all times, sometimes using words, is part of our love affair with Him.
A famous short prayer everybody knows is "My Jesus, Mercy." And the most famous short prayer in the Church, perhaps, is most beautifully practiced in the Eastern Catholic Churches, as well as by our Orthodox brethren, the Jesus Prayer: "Jesus Christ, Son of God, have mercy on me, a sinner." Some try and say it timed with every waking breath.
My man St. Francis de Sales says "Aspire to God with short but frequent outpourings of the heart." The Orthodox mystic St. Hesychois, in his Philokalia, says "The more rain falls on the earth, the softer it makes it; similarly, Christ's holy name gladdens the earth of our heart the more we call upon it."
It's not a mantra, because we are not using words to detach ourselves. It's not fruitless multiplication of words, because we aren't keeping count (we'd better not be). We're instead attaching ourselves to Him and most specifically to His Will, of offering whatever the moment is to Him, our resentments, our frustrations, our joys, our sorrows.
I want to grow in the habit of keeping a few beautiful words handy in the front of my mind, to sing or groan or cry them out to the Lord every time I think of Him.