The Benedictine monk, John Main (http://en.wikipedia.org/wiki/John_Main) had an image of the mantra in prayer, of it being laid down in the mind and heart like a feather on a pillow.
Last night, in a dream, I collected my own image. I was walking on the beach, barefoot, and the sea was grey and calm, and a mist rolled over it, stroking the waters as it came, touching so lightly.
As I walked on the beach, toes filling, un-filling with sand, I found myself saying the prayer of the heart (Lord Jesus Christ, mercy) in rhythm with the mist touched sea, in rhythm with my breathing: the same rhythm.
The emphasis, as always, was on 'mercy' and I woke with a calm sense of the compassion at the heart of things; and, still having the prayer say me.