I find standing balances to be tough, I think because I have flat feet. Then I get into trouble with anticipating the balance issues I am going to have as soon as the pose is called for. King dancer is my favorite standing balance, because there is so much else going on, I don't have time to worry about my fallen arches. 

I DON’T have fallen arches, and I can’t balance, either. I’ve practiced and practiced, and I’ve done every trick in the book, including focusing on an object, etc., and I still fall over. If you have any helpful hints on that score, I’d appreciate it.
Break the pose down into small segments. Start building balance by becoming steady with just holding the foot in the hand behind you with opposite hand raised over head (if necessary, stand with shoulder/hip at a wall). Once you become steady here (this may take days/weeks), move on. Stand with your shoulder/hip next to the wall and place a chair about 2-3 feet infront of you with the back facing you. While holding the outside foot up behind you, use the wall for support as you reach for the chair with the inside arm. Here you are fully supported and can work on pulling the rear foot up and back toward the wall behind you; this will give you lift. Remember to engage your core. Work on being steady here. Once you are, begin raising the hand from the chair. Go slow, inches at a time, building balance as you go (the wall is there for you). Once you have your balance here, your next step would be to start moving off the wall inches at a time. Remember, building balance takes time, patience and practice. If you take your time and honor where you are right now, it will happen!
Patience Practice .. Consistently and persistently will create balance
Debbie’s advice is right on. I’d just add that strongly engaging and drawing up the thigh muscle of the standing leg helps in all balances. When I started doing yoga, I couldn’t balance at all, even in tree pose. So you can improve.
Agree standing poses are the most difficult for me. Just recently discovered that doing Tree pose on each side for 3 miuntes has improved Dancers Pose. Tighting your stomach has also helped but agree Patience is Key Word.