The Yoga system that I believe works effectively is to practice of a few essential Asanas taken from the HaTha Yoga PradIpikA in order to first stimulate and then calm down the gross prANas (a term I've borrowed from SwAmi Rama in his book Path of Fire and Light). The choice of Asanas should be based on one's constitution (Ayurvedic dosha classification) and should address specific aches and pains that would otherwise disturb the sAdhaka in meditation. When these grosser prANAs are calmed down, then the body will not pose any problems when sitting for prANAyAma. Two key sUtras from the Patanjali Yoga SUtra need to be remembered when performing Asana. The first is
Sthira sukham Asanam
This means that the posture or Asana is intended to give the comfort (or pleasure) of stability. One should always keep this in mind when doing Asana. Clearly Power Yoga or Yoga and Pilates is not going to prepare you for meditation. If you feel you derive benefits from these, please continue, but do not fool yourself that you are doing Yoga. When the prANa-s are agitated it is useless to try and do prANAyAma, and it can even be detrimental to health.
The second verse to keep in mind when doing Asana is
Prayatna shaithilyam ananta samApattibhyAm
This means that one should progressively loosen the effort (remember passive volition) in the muscles and keep in tune with the Infinite. Gradually one become conscious of the flow of prANa, both in terms of prANa vAyu (breath) and energy in general. By focusing on the flow of energy, one gains a connection to the Infinite. In the more advanced stages of practice, one can actually visualize the pattern of one's breath (as noted by SwAmi RAma in his book Path of Fire and Light). This way the flow of energy from the nostrils, and also the feet and hands becomes clear. One can slowly feel the prANa coursing through the body as one performs Asanas. This prANa flow is connected to the flow of energy in the external world as well. This is the meaning of keeping in tune with the Infinite. In its true form, Asana Yoga starts to resemble Tai chi. It is not a vigorous cardiovascular exercise, but a gentle and conscious flow of prANa.
For this reason I could never see why people go to a group class for Yoga. Certainly one may take a few lessons to learn the Asanas if one is not proficient, but Asana practice must be done alone without distractions and at one's own pace. Everyone's breathing rate is different and in classes one cannot synchronize these differences in the pace of breathing properly. Furthermore, there are unnecessary distractions in group classes.
I found that B. K. S. Iyengar's Light on Yoga gives sufficient detail for the practice of Asanas by oneself. There was a set of about 10 Asanas in a booklet form by the Sivananda group that I also found useful. I can now remember only eight of these, and practiced in this order they are quite adequate. These Asanas are:
PaschimottAsana
HalAsana
BhujangAsana
DhanurAsana
ArdhyamatsyendrAsana
Supta VajrAsana
SarvAngAsana
ShirasAsana
One does not need to practice all 84 Asanas in the HYP, or the 400 odd plates in Iyengar's book. It all depends on where you want to go. After all, the real practice of Yoga should move from these external practices to the internal ones of dhAraNa, dhyAna and samAdhi. This is the real intent of Yoga. In sAdhanA you need to know exactly how much effort to spend at each stage. Too little effort and you do not gain the strength to successfully move on to the next stage. Too much effort and you can get stuck at the same level forever. One should not make the boat to cross samsAra (the ocean of existence) one's home. Otherwise the purpose of life will not be accomplished in this lifetime.