Since it seems like you have been “just doing it” and are still having trouble, let us use Tree Pose to know how to develop your one-legged balancing pose in successive steps.
Start up by standing solidly on both feet. Press the crown of your head up towards the roof and draw up your abdominal muscles into your spine. Draw your shoulders down and away from your ears. Anchor your Drishti (gaze) gently on the wall in front of you, or one spot on the floor. Experiment to find the focal point that makes you feel most constant. Set up a smooth flowing Ujjayi breath.
Next, focus on steadying and grounding the body. Transfer your weight onto the left leg and into the left foot. Then, raise the crown of your head up to the ceiling. Slowly draw the ventral muscles in towards the spine, pointing the tailbone (coccyx) straight down to the left heel. Lift the sternum.
When you are ready to take it up a mark, put the sole of your right foot next to your left fetlock, keeping just a hint of weight on the right big toe and opening the bent right knee out to the side. Exercise this until you feel confident here. Then draw the sole of your right foot up as high as possible on your inner left ham. Press your ham and foot into each other.
You can snaffle the raised foot in position with your right hand, extending the left arm out to the side at shoulder height. Or you can fetch your hands directly into prayer position (Namaste) in front of your heart. Fixed the eyes, relax the mind and breathe. If you fall out, do not judge yourself. Evoke the “so what” attitude, refocus your gaze, ground you, and simply do it again.
See also: www.sushilyoga.com
If abdominal muscles, legs, or weak ankles are keeping you from balancing, constructing muscle tone will be very helpful in the long run. Standing postures such as Triangle Pose (Utthita Trikonasana) and Warrior II (Virabhadrasana II) Pose develop leg strength.
You can also work on stability, core strength, and spinal extension, in Mountain Pose (Tadasana), standing on the toes or on one foot. The key here, as in all fundamental balancing poses, is to be grounded in your legs and feet, soft and steady in your breath and eye gaze, engaged in your abdominals and extended through the neck and spine.
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Inhaling, raise up on your toes; exhaling, lower down. Slightly increase the time it takes you to inhale and exhale so that you increase the time you are balancing up on your toes. When you are strong in this practice, add single, alternate arm rises coordinated with your breathing in and breathing out and rising up and lowering down. Finally, do the practice raising your both arms at the same time.
To exercise balancing on one foot in Tadasana, Unleash by engaging the same alignment and focusing principles already described for balancing on 2 feet. Transfer your weight over onto the right leg. Envision the weight of your body melting down into your foot, going deep into the floor. Envision your foot growing much wider and longer, the force of gravity anchoring your position. When you are ready, breathe in and raise your left foot one inch off the floor. Pause. Breathing out and set it down. Repeat until that feels easy.
Then continue, raising your foot a little higher, developing skill and confidence in small increments. When you wobble, check in and re-establish the alignment and focusing directions. If you fall out, so what! Take a full breath in and a long breath out, then start again. Be obstinate. You will get there and the world of balancing poses will open up to you. Do not be surprised if concentration, greater focus, and balance show up in other areas of your life we well.