The essential thing here is for that foot to be high and knee forced down as much as possible. And so, as pretty and and dramatic as the praying position looks, as much as you yearn to look like Buddha - ignore the temptation to pray and instead hold the foot securely in place with your left hand until the heel remains touching at least the bottom of your leotard/trunks when you let go of the foot.
Some yogis, after a year, still have to hold the foot up with one hand, putting the other hand to the breastbone in half prayer. This is perfectly okey, as long as you keep trying honestly to keep the foot up there without either hand and are not just being lazy :)
So, up with the heel, down with the knee, in with the buttocks, bear the pain, keep your balance for ten seconds!!!
Since your knee has been forced into unusual activity, treat it with care - shake it and wiggle it around a bit before reversing the pose.
The Tree Pose improves posture and balance and increases the flexibility of the ankles, knees, and hip joints. By strengthening the internal oblique muscles, it prevents hernia.
Read more about this poses benefits, pictures, video and tips from HERE
Drawings and info from "Bikram´s Beginning Yoga Class " Book, 1978.