In Gary Willis' Fingerboard Harmony book he talks about 4+2 left hand position. This was a new idea for me, and led almost immediately to a technical breakthrough that I am posting here to share. If you ever need to convince yourself that your left hand thumb needs to stay behind the neck, try playing a chromatic scale without using open strings. Unless you have huge hands, or don't mind jerking your whole hand up and down on every string change, you gotta use your thumb as a pivot. I think this is largely preaching to the choir here. 4+2 refers to a standard 1 finger/fret hand position with the addition one higher fret (to be fingered with the 4th finger) and one lower fret (to be fingered with the first finger). This hand position makes it a lot easier to play highly chromatic walking lines, and takes a lot of the guesswork out of coming up with fingerings for these. Although my thumb has always been in the 'right' place, I had the bad habit of letting my left hand collapse onto the neck such that the base of my index finger would touch the neck. Playing in the standard 1 finger/fret or in the 3 fret 'clam' position was OK, but as soon as I tried 4+2 I knew I had issues as it was very awkward to reach back with the index or forward with the little finger. Once I figured this out, and religiously made sure that only thumb and fingertips came in contact with the neck/strings, life became sooo much better. I notice much greater left hand precision (less buzzing), in addition to being able navigate that highly chromatic passagework more fluidly. So bottom line: watch your thumb AND make sure your hand doesn't collapse onto the G string side (on a 4) of the neck. My 2 cents.