Doing it all the time

One of the ideas underlying XP is "If it's worth doing, then we'll do it all the time". But doing something all the time can change the way that we feel about doing it.
I was reminded of this when discussing photography with Steve Purcell. He remarked that taking photos seems much easier when carrying a camera around all day: after an hour or so he no longer has to actively look out for worthwhile compositions as they seem to leap out by themselves. Which I think is similar to how it feels to practice Test Driven Development or Pair Programming.
If you only do these XP practices occasionally then they seem odd or unnatural, and it takes an effort to stay focussed and achieve results. (The developer is focussing on the act of development, rather than the code that she is trying to develop.) But if you do them all the time then you get into a rhythm: the tests seem to write themselves, it doesn't matter which of you is typing, and before you know it the card that you're working is on complete...

Postscript: a couple of people have mentioned the parallels between "the rhythm" described in this post and what sports people call "being in the zone" - the mind / body state that leads to heightened athletic performance. So now I'm wondering what a simultaneous PET brain scan of both members of a high-performance pair would look like. Any medical scientists out there looking for an off-the-wall research project?!