I found this link (above) while casually browsing for some article that espoused Paul's "don't practice scales" dictum. A lot of the points have easy generalizations to all kinds of practice. Its not a bad summary -- I believe kids can benefit from learning how to learn so to speak. The best teachers engender and reinforce good practice habits, even if not explicitly. Smart kids do discover some of these things about effective for themselves as well. Some things, like frequent review, just happen because the student is really interested ("for to the one that has, more will be given"...) And of course, as you point out, some just have the nervous system that allows for faster progress.