Yoga for Strength isn't about just doing poses to mindlessly get stronger. It is about directing your awareness and your effort so that you can apply strength effectively with less effort.
If you put your effort in the right place, you don't have to use so much of it.
For stronger yoga poses the first step is to create stability somewhere within your body.
The better you are at creating stability, the easier it is to get stronger with less effort.
If you are having problems developing strength it may be due to a lack of stability somewhere within your body.
Notice where you lack stability, so that you can work at creating it. Then you can get stronger with less effort.
Interested in learning how to feel and control your leg muscles?
Sensational Leg Anatomy includes a series of videos teaching simple muscle control exercises. The whole series teaches you to feel and control the major muscles of the legs from the hip bones down to the toes.
You can also get it as part of the Muscle Control Package(Learn to Feel Your Body)
Muscle control not only helps you to control your body, it also helps you to feel it making it easier to balance, improve flexibility and deal with pain and poor posture.
The dance of shiva is handy if you are interested in improving upper body balance and coordination.