Ardha Chandrasana or Half Moon pose is one of my most favorite postures. I love to balance on one leg and this one is great. In my opinion, if you're looking to master ardha chandrasana, the key is to look down about 90% of the times you practice the pose. Looking up is a just a finishing touch and adds nothing (physically). However, if you want a real mental challenge, look up and BALANCE.
If you have arms that are proportionately shorter than your legs and you're obsessed with getting a straight line through the spine, a block underneath your bottom arm works wonders.
If you want to challenge your balance even more and get a crazy strong quad stretch, go for Chopasana: A little shout out here on the iPad (pictured above) to my favorite, YogaGlo. Keeping me going! Here's a tutorial video for ardha chandrasana with one of my favorite teachers on there, Dice Iida-Klein:
There are countless to benefits to a balance such as this, but overall, as B.K.S. Iyengar said: