I found Richard Rosen's answer to this question (from the Sept/Oct 1998 issue of Yoga Journal) in Erich Schiffmann's Teacher Training Manual. Great advice. — BK
"Like most yoga questions, this one can’t be answered with a neat either/or reply. Yoga practice has two elements. One is discipline—a voluntary submission to a traditional, goal-oriented system. Such a system helps us focus our energies, but it can also limit our possibilities and eventually obstruct our evolution.
The necessary counterbalance to discipline is spontaneity. Self-expressive, experimental, open-ended, and playful, spontaneity is the joyful manifestation of our innate wisdom. In The Call for the Master, Karlfried Graf Durkheim calls this voice the 'inner guide,' the 'primal knowledge... of the inborn way in which we can fulfill our destiny.'
I think it’s important at the outset of practice to commit to a structured asana program and stick to it as faithfully as possible. Doing this lays a solid physical foundation for future work, and begins the process of yoga: quieting the vritti ('fluctuations') of consciousness. Over time, as the voice of our inner guide becomes less obscured by these fluctuations, we can allow it to spontaneously modify more of our daily routine.
“Old man, stiff man, weak man, sick man, they can all take practice... but only a lazy man can’t take practice.” — K Pattabhi Jois
I don’t mean to imply that beginners shouldn’t trust their natural inclinations. It’s just that sometimes it’s hard to distinguish between sluggishness and plain old laziness, one of the nine obstacles to self-realization listed by Patanjali (Yoga Sutra 1.30). On a slow day, you can often jump-start your regular practice with something vigorous like a Sun Salute. If that doesn’t work, then gracefully acknowledge that you need another course of action. Try to listen for whatever your guide may suggest; after all, according to Kashmiri Shaivite Swami Savitripriya, a recent commentator of the Yoga Sutras, 'Knowledge based on personal experience alone will make you free, not dogmas or beliefs systems.'"
Don't know where to begin?
Try one of my Foundation Series videos, they're short, well-rounded sequences designed for daily use.