First, I believe that there are only two ways to learn how to maintain a flat left wrist. The first way is that you know you need it because you've stumbled upon The Golfing Machine. The second way is that you've accidentally figured it out. The latter column is where the majority of golfers with flat left wrists fall.
Now, I was fortunate enough to have found the correct information almost from the outset of taking up this game. I had only a few years, at most, to develop bad habits. I was lucky enough to learn from those who really knew their stuff way back in fall 2000 when I began with the book.
That said, I've developed an idea regarding the whole 'flat left wrist' concept. After going to St. Augustine and learning from Lynn and Chuck, it really sunk in. It was already firmly in my head before, but after the workshop even more so. Over this past summer, I can probably count on both hands the number of times I made a stroke with a bent left wrist. And that isn't that large of an exaggeration either.
Once you 'get it in your coconut' that you will not hit good shots without having a flat left wrist, things really take off. Read that a few times until it really sinks in. I will not consistently hit good shots without having a flat left wrist. Now go out and practice it, starting with short wedge shots and progressing your way up to the longer clubs.
Once you have it dead set in your mind...your Computer won't let you do otherwise! When you know that you need a flat left wrist, your mind just won't let it bend! That's the way I see it, and my results back it up. When your mind is entirely wrapped around a concept, doing otherwise is 'not allowed' and your Computer knows that. Give it a try.