Tadasana is an important grounding point for exploring Vrksasana. Your standing leg must be as steadfast and firm as it is in Mountain Pose in order to support the graceful balance of Tree Pose, another pose listed as 1* on the scale of difficulty.
In Light on Yoga, Iyengar provides surprisingly basic instruction for a pose that I find anatomically complex. Your ability to stand firmly on your left leg in Vrksasana is compromised by the pressure of your right foot pressing against your left inner thigh. The left hip point may splay out, pushing your left hip and leg out of alignment. The splaying of the left hip, combined with the external rotation of the right upper thigh, might then lead you to stick out your butt, thereby tilting your pelvis forward, then arching your lumbar spine, then protruding your rib cage forward, then jutting out your neck… you get the point.
When I take Vrksasana, I find it helpful to focus on pressing the inner thigh of my standing left leg into the sole of my right foot. When my left thigh presses into the sole of my foot as much as the sole of my foot presses into my left thigh, I can draw my standing hip under me and achieve greater stability in the pose. I like to imagine I’m holding a precious diamond in the space between the sole of my right foot and my inner left thigh, and that the pressing of my foot and thigh against each other is the key to keeping it from dropping. Hey, whatever works, right?
Iyengar mentions that Vrksasana “gives one a sense of balance and poise,” but many people have trouble balancing in this pose. While Light on Yoga doesn’t provide guidance on how best to maintain balance in this asana, it helps to find a dristhi, an unmoving point of focus on which your eyes can rest. Gazing upon something stationary may improve your stability.
You can also have some fun with Vrksasana by exploring it as a variation on other poses. In my personal practice, I enjoy taking a Vrksasana variation of Side Plank, then moving into a Vrksasana variation of Caturanga Dandasana. When taking this variation, though, before moving into Urdhva Mukha Svanasana (Upward-Facing Dog), reposition yourself so that the front sides of your legs face down for Urdhva Mukha.