Robuki, or blowing ro – blowing the note on a shakuhachi with all holes covered – is a common practice routine, and some say that every shakuhachi player should practice robuki for ten minutes a day.
But it’s not just ro; you can practice -buki with any note. Take tsu no meri, the difficult note that is a half step above ro. As my teacher said when I first encountered this note, it is the “soul of the shakuhachi.” But it’s a mighty hard note to play, and it’s probably the biggest hurdle for beginners after they learn to make stable sounds.
So as I have progressed, I’ve managed to play tsu no meri much better, but it is important that it is the right pitch. So my teacher recommended, in my last lesson, to play tsu no meri buki, and especially to start by playing another note: either ro, or re then tsu, to ensure that I get the correct interval and the proper pitch.
I often spend a half hour just playing long notes. It’s good practice for the embouchure, and it’s really good for the breath: not only to improve breathing capacity, but also to ensure that my blowing is even and that, as much as possible, notes sound evenly.
Honkyoku 
I am exercising tsu no meri since about half a year. It comes still kind of muffled and with unsteady pitch. Funnily it works in the kan register. For me tsu no meri is the most challenging note on the Shakuhachi.
It is certainly the most challenging that I have encountered so far. But I recently made a breakthrough, and it’s starting to work a lot better. In my experience with this instrument, each challenge seems to resolve with time and patience, rather than trying to fight through difficulties.