What if you're practicing wrong? Then you get very good at doing something wrong. If we don't get good instruction, then we don't notice when it's a little out of round. Surrender yourself to your teacher. That's doesn't mean you turn over your life to the teacher - you don't want a guru. You have to keep the autonomy within yourself. You are finally the ultimate authority on your own practice.
Source: Mastery: The Keys to Success and Long-Term Fullfillment (Plume)
Contributed by: ~C4Chaos