I heard Perlman on the radio telling students to listen to their own playing, which he said is not as obvious as it seems. When my practicing is going poorly it's almost always because I am not really listening. The comment about listening to the piano itself is also very valuable. I was working on the Beethoven Opus 110, years ago, and on a long soft chord near the end of the 1st movement my teacher remarked, "You're counting like mad, aren't you?" She was exactly right, and she told me to listen to the decay of the chord, and it would tell me when to go on. I tried it, and could hear the changing voices in the chord indicating the exact time to move. It was a marvelous moment, although I have not acted on the experience as much as I should. I suspect that the most experienced players do this almost without thinking, and even when the movement of notes does not seem to give pauses long enough for most of us to listen so closely.