What's the difference between Christian prayer and mantra?
Jesus warned in the Sermon on the Mount, "Do not be like the Gentiles who think that they will be heard because they heap up many words. Rather, when you pray, say, 'Our Father in heaven, hallowed be your name,'" and so on (Matthew 6:7ff). So Jesus is the one who warns us against mantras, that is, more or less, the mindless repetition of words thinking that God is more constrained if we say something more often.
The difference is that we are coming to a heavenly Father who says to us, "If you know how to give good gifts to your children, how much more will your heavenly Father give good things to those who ask him!" (Matthew 7:11). So we're coming with the confidence that he wants to hear and that he is inclined and prone to answer. We don't have to twist his arm behind his back.
We come expectantly, reverently, and simply, and we do what he says. "Ask and you will receive, seek and you will find, knock and the door will be opened" (Matthew 7:7).
So I think it's the character of God that keeps us from sinking into a kind of magical mode of counting beads or repeating phrases. We are coming to a father, and when you go to a father you don't go through mechanical magical motions. You look him in the face and say, "I love you. I know you love me. Here's what I need, Daddy. Would you please help me?"