I have found standing head to knee and standing bow to be the toughest ones for balancing. Reason is that you hold the poses longer than other balancing poses. According to Bikram’s 2007 book, you hold these poses for 60 seconds. Balancing stick is supposed to be only 10 seconds, again according to Bikram’s 2007 book. I have found the studios do a good job on the former poses in sticking to the time rule. But they hold most of the other poses too long. Perhaps because they cannot squeeze in the pose instructions in only 10 seconds.
I use those techniques for all balancing poses to some degree and they help. But even after doing this style of yoga for 15 years, I will occasionally fall out of some of the balancing poses. So it depends on the kind of day I am having as well.
Perhaps the idea to focus on one spot helps one to keep the mind focused. When my mind wanders my balance decreases.