Having a personal practice that you do regularly is what T.K.V. Desikachar calls "the heart of yoga." It can lead to both physical and mental insights.
Students learn quite a bit about their alignment in various yoga asanas during yoga classes. However, putting together a series of poses appropriate for your goals, and a sequence that extends your capabilities while preventing injury, is beyond most students' understanding. Even yoga teachers frequently work with another teacher to benefit from an objective "view from outside."
Typically, in a private lesson, a student describes his or her goals, limitations and the amount of time he or she can devote to a practice. With this information, a teacher tailors a yoga practice to fit the student's time and goals. The student then comes back once or twice within a few weeks of the initial session to have alignment and breath fine-tuned. Typically, a student then keeps the practice for two or three months before returning.
A student grows with a daily
practice. After a few months, the student's practice should be reviewed
and a new sequence developed to reflect the student's increasing abilities
and changing needs.