As mentioned in the previous tip, a common feature in honkyoku is contrasting sounds that have the same pitch but very different tone color because they are played with different fingering and/or head position. Here are a few important same-pitch pairs to practice. The idea of this exercise is to go back and forth between the two notes making sure that you're keeping the pitch identical.
Tsu dai meri - ro: Practice in both otsu and kan registers, going back and forth between these notes, articulating each one with a slight hit on the second hole. It requires a relaxed neck to get the meri deep enough and the kari high enough.
I (otsu) - ro (kan): This pair does not require any head movement, but often students play i flat. On some flutes it takes a bit of concentration to get the i right.
Ichi san no u (kan) - re (kan): Ichi san no u, called ru in some lineages, is played with holes one and three open and the head lowered quite a bit. It is the same pitch as re but with a very different tone color. Go back and forth, keeping the head loose and hitting hole four lightly for each note.
Re no meri - tsu (otsu and kan): Play a full re no meri, with holes one and two partially covered and your head lowered. Hitting hole two, let your head swing up quickly for the tsu. When you go back to the re no meri, hit hole four.
Of course there are other same-pitch pairs like this, and you can do this same back-and-forth exercise with all of them.