When you want to practice for, say, 2 solid hours a day, how do you stay productive throughout the practice session? I find myself going off and noodling after a while of practicing something monotonous like a lick in 12 keys or scales, etc. and this seems to be a waste of actual practice time, set aside the fact that it's fun to do.
Perhaps the most obvious thing to do would be to consciously decide to take breaks at specific intervals. Then you can set a time-limit on it. The 5min per half-hour (or whatever pace you decide) of free play will be sweeter for having anticipated it. I can be a frequent reward for diligence during the work portion.
Edit: I found some psychobabble to back this up.
[I]t is very useful for mental workers to practice a short relaxation every hour; short, so that the needed and right mental tension is not lost, and yet the body is freed from excessive tension. Such short relaxations depend on one's physical condition: if very tired, relax on a couch whereever possible; if not, stand up and move about. In both methods include deep breathing, but such relaxation should not be for more than five minutes. It provides a useful break, helps toward sustained work--a "second wind", so to speak; and those who are tense relax and those who are slack are helped to become active. -Roberto Assagioli, Psychosynthesis, 1976, p. 160.
I try to stay task-oriented. Before I start playing, I always identify a goal, whether it be focusing on not screwing up a note, or trying out a different phrasing, or working on pedaling , or finding a different fingering. After I stop playing, I reflect on what I just did and what I could have done better-- was a slur not fluid enough, could I have made the dynamics a little more distinct, was I even playing the right notes.
For me, this work process breaks down at about the 40-60 minute mark, and I need to take a breather, like luser droog recommends.
I would say : do NOT pratice for solid hours. Why do you do so ?
To keep your attention up, practice for any number of 30 minute sessions.
You will be amazed by the improvement of 2 x 30 minute sessions over a single 2 hour session, the day after.
Summary: : Extended sessions are NOT the way to go for increased performance or productivity. Cut them into small chunks !