So yesterday, I managed to squeak out my first F5 in years. I've never really had a solid F5, nor an even passably useful one, but back in about 2002 I was actually able to squeak the note out a little bit from time to time. I haven't heard anything from it since then.
How did I get it? I've been doing my range exercises faithfully, "do-re-mi-fa-sol" starting on tuning note Bb and moving up by half steps until I can't take it anymore. When I started these exercises, I managed to get up to a C before my chops gave out. Slowly, doing these every practice session, I've been making progress. Once I got to the D, the Eb came easier, and the E was almost immediately ready, and then yesterday, the F5 just popped out of nowhere.
And then, today, it's gone again. I suppose I will just have to keep trying to tame that elusive F5.
The key to taming it for me, I think, is building up stamina in the upper register. I get tired extremely quickly when playing anything above F4. I cannot make it through even half of one Bordogni etude taken up an octave. So in order to tame the F5, it seems that I need to tame the E5, the Eb5, the D5, the Db5 and the C5 first.
...so much more work than I had in mind, but you know, playing the trombone isn't easy.