Tree pose is actually a very challenging posture when performed the right way. Ideally, from the side view, your two knees should be in one line. Many people are able to hold the foot up in place by cheating and letting the hips/butt stick out with a swayed back. You need to squeeze your butt muscles and push your hips forward, while at the same time, pushing your knee gently down and back, to get everything in one line from the side. Continuously stretch your spine up to grow taller.
The following by tracysfoodandthought.blogspot.com shows some good pictures and tips for opening your hips in tree pose by squeezing your glutes.
The pose I demonstrate above is "Tree pose". In Bikram's yoga practice
there are a number of poses that help open up the hips.....when done properly!
I can't tell you how many times I watch people do this pose
improperly.....about 100% of the time! Even when the teacher clearly
instructs to NOT have a "duck butt", guess what? Apparently no
body knows what a "duck butt" is, OR their hips are so tight they can't
help it! But if that's the case then you should take advantage of the
use of your opposite hand and hold onto your foot to pull your bent leg down and back. The "goal" is to have both legs in one line, while pushing your hips forward to open them up!
A good clue to whether or not you are doing this is to pay attention
and contract your glute muscle (squeeze your butt muscle!) In the above
picture you can see I'm still a good six inches away from that!
Tree pose, in Bikram's yoga, is NOT a balancing posture. Sure,
you have to balance, but it's a hip opening posture, and your standing
leg/hip/glute should be contracted...in other words, SQUEEZE your
butt muscle! If you cannot squeeze your glute then you are not in the
posture. Everybody is in a hurry to put both hands in prayer and look
pretty! In the bottom photos I demonstrate my hands in "prayer
position" and then only one of my hands in prayer, while the other holds
onto my foot, while pushing my bent leg down and back, and while
squeezing my glute muscle and lifting up on my knee cap of my standing
leg/hip. This does two things. Squeezing my glute helps to open my hip
flexor to bring my hip forward, and lifting up on the knee cap of my
standing leg ensures that my standing leg is straight. Oh...and I also
try and make myself tall (like a tree) by lifting my rib cage and
shoulders, up and back, visualizing the top of my head being pulled up
towards the ceiling, while on the opposite, rooting down through the
floor with my entire foot (roots like a tree).
In my opinion it's much more important that you open up your hip than it
is to balance with both hands in prayer, looking pretty, with a duck
butt! But that's me. Eventually I want to get both of my legs in the
same line, while having both hands in prayer, with full extension in my
hips, now that would really be pretty! Pretty and correct according to
the "dialogue". And I'm so close!