In yoga for strength an important consideration for smooth (and relatively effortless) advancement is the idea of progressions, gradually making an exercise more difficult.
In weight lifting the equivalent would be gradually adding more and more weight.
A prerequisite for both strength and flexibility is stability.
Stability generally requires using muscles against each other with the result that the opposing muscles are strengthened. And so practicing stabilizing parts of the body is actually a strengthening exercise in and of itself.
But in general stability is the practice of making a part of the body resistant to change. Then it can be used as a foundation for another part of the body to create change.
For more on stability (and strength) you can read exercises to improve stabilty.
One way to improve strength while doing yoga poses is to practice moving in and out of yoga poses, particular those where the arms or legs bear the weight of your body.
One of the harder exercises for the arms is lifting up into wheel pose.
You can practice holding this pose to develop strength or practice lifting and lowering repeatedly.
Details on learning how to lift the head are covered in the pdf Making Wheel Pose Easier.
For strengthening the legs, one of the more difficult exercises is the one legged squat or "pistol" squat. To work up to this, first work at doing a regular squat on both legs.
Another useful preparation exercise is the seated get up knee exercise, demo'd in this video. Note the possible importance of activating the calf muscles and the back of the knee for knee stability.
From standing, see if you can lower to half way (bottom left), then stand up again. From a full deep squat (below right) you can also lift into the half squat.
In both the deep squat and the half squat you can then work at shifting weight on to one leg. Work towards this gradually. Then once you can get all of your weight onto one leg see if you can lift the other leg.
In a deep squat, if you can get all of your weight onto one leg, then reach the other leg forwards.
You can first practice standing up from this position, then lower using both legs. Once you are comfortable standing up with a once legged squat, then work at lowering down on one leg.
A pose that can be used to work on the back of the legs and or the arms is reverse plank.
To strengthen the hamstrings try these hamstring strengthening yoga poses.
Hip strengthening exercises uses some of single leg standing exercises to strengthen the hips (and knees) and improve pelvic awareness and control (relative to the thigh).
Knee strengthening exercises aren't so much about particular poses as it is about activating the knees in a variety of poses to make the knees stronger.
The back can be divided into an upper and lower half. Generally the upper back is the back of the ribcage while the lower back is the part that connects the pelvis to the ribcage (the lumbar spine.)
Upper back exercises include exercises for becoming more aware of the thoracic spine and ribcage and also training the spinal erectors to bend the ribcage backwards.
Standing Exercises for Low Back Pain (for dealing with low back pain and helping to prevent it) include a mix of standing low back strengthening and hip strengthening exercises.
One of the simplest exercises for the abs is to slowly sit up and balance on the butt from a laying down position. With arms and legs reaching up, slowly reach arms and legs forwards. Then when you feel your center over your buttocks, slowly sit up.
To make it easier grab onto your thighs.
Then lay back down slowly and in control and repeat.
A progression is to roll up to a squat. (First do it fast to get into the squat, then see if you can slowly roll up into a squat.)
Then you could also try rolling up into a one legged squat as above.
Frictional muscle control helps you to strengthen your arms and legs.
If you aren't very strong, you'll learn how to get strong
and improve body awareness at the same time.
And you'll learn to use your body intelligently, even as you strengthen it.
Learn Your Body with
Frictional Arm and Leg Strength