Breathing Out While Waiting
I didn't think the D major scale in Barber would be a big deal.
A couple of years ago, I was playing this bit from the Barber of Seville Overture in rehearsal.
I didn't think the scale (D major for Horn in F) would be a big deal. The conductor wasn't even taking a particularly fast tempo. But I fumbled the fingerings on the way up, resulting in a crash-and-burn. I mentally shrugged it off, and the conductor did not go back and repeat the section.
The same thing happened, however, in the next rehearsal, which concerned me. So, the next day, I put the part on my stand and tried to recall what I was doing, thinking, and feeling at this point in rehearsal. I realized that I hadn't really felt ready to play at the moment the scale arrived.
How could that be? There is a nice long rest before the scale. All I had to do was to count 12 measures of rest, twice. I began to suspect that was the problem: too much waiting time, enough to get physically and mentally fidgety. I didn't feel quite ready for a big breath in just before the scale, and it seemed like a timing problem.
I experimented with a planned exhalation, starting several measures before the scale. Specifically, I planned to start breathing out in the 9th measure after 8, and to continue breathing out until a couple of beats before the scale. At that point, I would be guaranteed to be fully ready to take in a big breath.
I find that timing is not critical when breathing out. We are almost always ready, at any point, to breathe out, because our lungs are rarely empty. By breathing out slowly, against slightly pursed lips for a bit of resistance, it's easy for me to breathe out for a long time. And it is just active enough to keep my body busy, in a productive way, and to quiet my mind, while waiting to play the entrance.
This planned long exhalation worked well for me with Barber, and I have continued to have regular success with it in other situations.