I have a violin student who came to me looking like a disaster. He's improved dramatically since he started coming for lessons, but still wasn't as consistent as I needed him to be, technique-wise. So last week we did some kinesthetic visualization ("feel your fingers before you play") and the improvement was marvelous. That was his work for this week.
Today he came in looking like a professional violinist. And proceeded to play double stops like nobody's business. As I watched him play I was struck (yet again) with how amazingly cool it is to watch this transformation. I am very, very lucky.