Adapt instead to reap maximum benefits
Since no two bodies area alike, no two bodies will look identical doing the same postures. Know the purpose of your practice before you begin each session. Examine how this purpose fits in with your larger goal of reintegration, and make the changes that will keep this immediate goal consonant with the practice. For example, if you have had a stressful day, your goal for an evening asana practice may be to calm down. On the other hand, you may wish to design your practice to wake up your body and mind in preparation for evening activities. To know your purpose, you need to reflect on how you feel at the moment and how you would like to feel at the end of your session.
You must continually redefine your practice on an ongoing basis, to ensure that your goal is realistic and the course for achieving it appropriate. It is easy to fall into habits, especially with regular or daily activities such as asana practice. When this happens the practice may become sterile, and the sense of vitality and progress may disappear. The mind wanders, focus on the body and breath is lost, and what was originally asana practice becomes simply a set of mindless exercises.
Any practice aimed at improving yourself must reflect the constant flux of daily life. Each day, your body, mood, schedule, environment, and so on, will differ. In light of this, on certain days a longstanding goal or practice may require some reflection or change. You accomplish this change through adaptation of both your practice as a whole (the specific poses you put together), and the poses themselves. This is the art of adaptation.
The best indicators of whether you are working too hard or not exerting yourself enough are the breath and your own heightened awareness. Your constant observation and self-reflection will signal when a change is necessary. For example, when you practice certain postures every day to the exclusion of others, they may become addictive. It may be worthwhile to stop doing these particular postures, and to reflect, instead, on the factors that might be relevant in your attachment to them.
If you have physical symptoms such as those resulting from illness, rest may be more important than practice. In the case of injury, you may need to interrupt your practice to allow for healing. Often, in such instances, simple breathing exercises will accelerate your recovery.
An appropriate asana practice includes the right amount of work at an intensity that challenges, but does not injure or fatigue the practitioner — again the balance of steadiness and comfort.