I might just add that, having tried a couple of the variations of the Jesus Prayer Ms. Pennock suggests, I've gone back to the basic, "Lord Jesus Christ, have mercy on me, a sinner." Every once in a while I'll add something like, "and deliver me from ........." (pride, or impatience, or whatever).
I've found the value in this is that it helps make me consciously aware of where, at a given moment, my spiritual battle is taking place.