If you ask any professional what amateurs do wrong the most you will almost always get two answers. The first, which we alrady covered, is that amateurs never take enough club on approach shots. The second will be that they do not play enough break on putts. Studies have shown that people usually see 30%-50% of the actual break, this includes professionals. Obviously people are making putts so compensations are being made.
There is a very simple reason why professionals two putt less than amateurs, they leave their first putts close to the hole on a far more consistent basis. This is because they play more break so their putts are rolling towards the hole at the end of the putt, not away from it. This seems like a simple notion but it costs people strokes in every round. Next time you are on the course pay attention to how many putts are missed on the low side and you will be surprised.
Next time you are on the putting green find a breaking putt of 15 to 20 feet. Hit five putts and try to keep them all above the hole, if not in. Within a couple practice sessions you will start to notice that you are making more putts and your second putts are much shorter.