I noted on the surrendering to grace thread when I posted this same link to a very good discussion of prayer of the heart-of which the "Jesus Prayer" was 1 example that I would start a thread just re this in the event anyone wished to dialogue re it further. In more ways than 1 I believe the "heart" is where the rubber meets the road.
http://www.hridayamyoga.org/papers/EssenceofPrayeroftheHeart.pdf

http://www.hridayamyoga.org/papers/EssenceofPrayeroftheHeart.pdf