The best example of this for me is playing drums. It's a very physical thing to do and extremely easy to get caught up in the fun of it. You find yourself playing very hard with a lot of tension everywhere. The problem is that it's very difficult to do this for long periods and if you want to play a series of fast notes accurately, it's counterproductive. So, I'm constantly telling myself to play as soft and loose as possible until one day it hopefully becomes automatic.