Learning how to practice. I'm going through some rather rigorous hands-on training where more than half of the students end up failing out. It distresses me to see everyone wasting a full hour running an entire problem when the particular action they actually need help on takes 10 minutes, tops. Practicing efficiently is one of the best things my father taught me. Graduate+ level musicians are bound to have great tips for this. Of course, effective studying is in many ways similar, but it seems more intuitive. That is, no one could possibly be stupid enough to study for a literature exam by sitting down reading the book cover to cover several times.