I find myself drifting in and out of various "modes " of concentration when I shed it up...Each has its own particular area of directing all my concious awareness...to either 1.) Concentration on purely what I am playing sounds like. 2.) What I am currently playing feels like (physically I mean) or 3.) what it is that the phrase, chord, exercise or whatnot looks like. Or Lastly 4.) How does what I am playing relate to me emotionally (Sometimes the thing I am practicing will not resonate with me at all and other times it just make me burn all over inside-either instance is fine but just good for me to be aware of) I find that the more awareness I develope concerning these concentration channels that I gravitate towards, the more efficienty I learn, memorize, tighten, and refine whatever it is that I am playing/practicing.