Nadine of Just Breathe left a nice comment last week, in which she decried the dangers of "attainment yoga." I really like that term--attainment yoga--it's so evocative and describes exactly a practice that is all about results rather than process. It's such an easy trap to fall into; I hear it often from my students ("I really wish my head would touch my knee") and myself ("It would be so cool to be able to drop back into Urdhva Danurasana [Wheel Pose]"). In longing to achieve some difficult pose(or even impossible, depending on one's body), we force and strain and over-do and completely lose the whole point of a practice.
And yet, there's something to be said for developing your yoga skills. Suzi of Yoga Like Salt (I wish we'd see more of her posts--such a sensible, thougtful writer) has a really nice New Year's article about setting a yoga goal for the year and working to reach it: Yoga Incrementalism, she calls it. I think this is a good approach. Asana practice is a physical journey, after all, and asking a bit more of our bodies as we do the work makes sense. Breaking down a pose and figuring out what part requires strength, what part requires flexibility and how to achieve those ends is a reasonable way to deepen your understanding of the work of yoga.
So, I guess it boils down to moderation. Work to improve your practice physically, but make sure to be respectful of your body's ability. Just because it looks good or difficult in a picture, doesn't mean it's an appropriate action for you. And don't forget that there is as much work to be done mentally as there is physically--always practice vairagya (non-attachment) with the superficial stuff; the head on the knee is icing, tipping the pelvis and getting a good stretch in the backs of the legs is the cake.
Here's to increments; a dense, rich asana cake; and moderation in all things (especially, moderation--says the husband)!