I haven't taken anything practical away from this forum. I'm fortunate to have a superb teacher who addresses my concerns then corrects them in a fraction of the time it takes to read a page of text and not really do anything afterwards.
The "best thing I have learned" is pretty much anything I have learned, but if I have to single out one thing, it would be how go about learning new repertoire by using well-thought out practicing appraoches, particularly mental practice.