I'm increasingly convinced that two of the most crucial factors for singing success are mastering appoggio and the passaggi. Today, I'd like to offer some tips for the latter.
Our goal, of course, is to sing smoothly from the lowest note we can sing to the highest--for the register boundaries to be imperceptible. I tried, for quite some time, to wipe away these boundaries by simply ignoring them, and of course that didn't work. I finally reviewed my various texts on vocal technique, and realized that they all contained remarkably similar information about where a baritone's passaggi would lie: the consensus is that the average primo passaggio is around B3, and the average secondo passaggio is around E4. Once I started doing scale work with full knowledge of what pitches I was singing [I do not have perfect pitch], I realized that, lo and behold, I had passaggi at the predictable places!
Once I started recognizing the passaggi rather than ignoring them, my voice began to smooth out and my range expanded. And indeed, my goal of smooth transitions was realized. This is crucial: smoothing out my register transitions was only accomplished when I paid MORE attention to the passaggi, not LESS.
I'm a firm believer that a relatively brief period of intense focus on the passaggi can build the skills of smoothly transitioning, such that the singer can then turn their attention elsewhere and not be pre-occupied with the passaggi forever. Like any skill in singing, a short period of intense focus can lead to a sustained period of that skill operating well in the background. [Passaggio management is one of the many technique skills that we'd like to become habitual such that we can be in the moment during a performance and focus on the art of singing, rather than the craft of singing.]
I now help my students to become aware of the passaggi by highlighting their scores:
For women, any notes at or below the primo passaggio are highlighted green, and any notes at or above the secondo passaggio are highlighted yellow.
For men, I only use one color: yellow, for anything at or above the primo passaggio. The reason for this is that, in my opinion, the laryngeal tilt, vowel-focusing, and airspeed increase that men do starting at the primo passaggio simply continues through the secondo passaggio and beyond. [Many men have only a handful of pitches above their secondo passaggio.]
Once they've highlighted their scores, they have a visual cue to allow the voice to shift registers [I use the time-tested analogy of smoothly shifting gears in a manual transmission car]. I do not have them highlight every score, but some students find this practice so helpful that they transfer it to many, if not all, of their scores.
So here's an example from an actual score, Duparc's delicious "L'invitation au voyage." The highlights are for baritone passaggi.
When I sing the above passages, mindfully moving into and out of the passaggi where indicated, all is well. If I am late to a register change, poor sound and vocal strain ensue.
Here's an example for a soprano, using the green + yellow system described above. Note that I'm using the standard passaggi of primo around E4 and secondo around F#5. For this reason (among others), I find the Estelle Liebling Vocal Course invaluable, since she writes out the exercises in the actual keys one would sing them in, and therefore the student can be looking at precisely what they're singing in progressive keys. This is Exercise 22 from the soprano volume of the Estelle Liebling Vocal Course.
Empirically, I have found the vast, vast majority of singers to have "conforming" voices, meaning in this case that their passaggi are identical to or very close to the accepted norms. Perhaps there's a certain type of student who is drawn to being "different," who desperately wants their voice to operate in a unique fashion. Regardless, upon further investigation, all but a few voices I've taught conform very closely to the widely-shared average passaggi below.
My best advice is to face the passaggi head-on. As mentioned earlier, a relatively brief period of close attention to this issue nearly always solves a host of correlating issues, and also creates a habit base that can make this behavior nearly automatic.