It's practice, practice, practice. Don't focus too hard on the details; relax and let your body and mind make the minor adjustments; don't tense up, stay loose. One thing I did when learning to track-stand was to lean with one hand against a big tree while on my bike with the other hand on the brake, then I experimented with the balance points until I could let go of the tree for longer amounts of time. Did get a few bark scratches on my arms but it worked well for exploring different points of balance.