From the book “Art & Fear” :

The ceramics teacher announced on opening day that he was dividing the class into two groups. All those on the left side of the studio, he said, would be graded solely on the quantity of work they produced, all those on the right solely on its quality. His procedure was simple: on the final day of class he would bring in his bathroom scales and weigh the work of the “quantity” group: fifty pound of pots rated an “A”, forty pounds a “B”, and so on. Those being graded on “quality”, however, needed to produce only one pot — albeit a perfect one — to get an “A”. Well, came grading time and a curious fact emerged: the works of highest quality were all produced by the group being graded for quantity. It seems that while the “quantity” group was busily churning out piles of work – and learning from their mistakes — the “quality” group had sat theorizing about perfection, and in the end had little more to show for their efforts than grandiose theories and a pile of dead clay.

Analysis Paralysis.  I sometimes tell a student that while mindless “practicing” (which isn’t really practicing, it’s just putting in time)  is counterproductive, dangerous and generally bad, mindful repetitions, playing something over and over, observing along the way, making minute corrective changes as you play, over and over, can lead to better results than highly sophisticated, well-thought-out ideas that never make it out of your head.  

There is a difference between making clay pots and practicing the piano:  if your first clay pot doesn’t turn out to be a masterwork, you can just set it aside and go for another one.  When you practice, which is an athletic activity, every repetition has the potential – for good or for bad – to form a habit, to cement one certain way of playing.  There’s nothing – well, not much – more frustrating than finding out, after having played something numerous times, that you practiced a mistake – wrong note, wrong fingering, wrong rhythm, wrong motion.  It tends to become ingrained, and thus a pain to undo. 

Still, there is something to be said for just jumping in and doing it, mindfully, listening, paying attention to what you just did, over and over, correcting as you go.