Michael R. Thornton, principal horn of the Colorado Symphony replied to my letter about a special Strauss program (Don Juan, Till & Heldenleben),which I have done several times under Sawallischs batton when on tour. My answersmight be of interest for some list members. So I respond here on the list. 

To make it more hard, well, we had a dress rehearsal at 7:00 PM, runningthrough Don Juan, Till & 60% of Heldenleben, a rest of 30 minutes & theconcert at 9:00 PM. The encore piece was Mastersinger Ouverture. 

Yes, this concert, just ten years ago, needed all my experience. In such aprogram you have no time to think about mental stress. You have to masterthe stress for your body, to be seated such a long time in the heat. We had 35 centigrades. You need to know, where & when you can save lip power. You have to know how to make the task easier for you. There is nothing to save in the Don Juan except a few measures of unison passages, where you can reduce your "audible output" (named fortissimo !). But if you do, mostly the section does also, resulting in a complete loss of the horn sound required at that spot. So in fact, no chance to save. But playing on the F-side can help, as the many co-sounding waves (overtones) enforce your sound, thus to arrive "in good health" at the end of the piece. The Till after Don Juan is not so smart, as the initial solo requires a very relaxed embouchure. It should sound easy & weightless, while the later passages require the real forte also. Remember, I had played the whole program at 80 % just before the concert during the rehearsal. But here I could save indeed, leaving off most tutti passages but few here & there to try out the acoustic circumstances, but mostly concentrating upon the solo passages. 

Then the Heldenleben monster. Here the same might apply as above. Use a lot the F-side for relaxed playing. If your mouthpiece bore is wide enough, you will not find that much resistance against the tone production. A narrow bore mouthpiece will kill your embouchure. Horizontal pressure is much better during the fortissimo passages than vertical pressure, which cannot be avoided all times. 

Well, at rehearsal number 78 one can "reduce speed" a bit & let the section do the great unisone fortissimo, but do not forget to enter with the high Bb as a pick-up the measure before 79, reduce speed again the following two measures & enter softly with the Db solo entrance the measure before 80.

Very relaxed on the F-side. Then the solobeginning (in E) at 81. Enough time for breathing. Again start with the F-side, switching to the Bb-side for the (played) F# in the 5th measure of the solo, immideately switching back to the F-side for the half note A (played), the scale all F-side & the climax G# on the Bb-side. The following passages are demanding but not exhhausting as there are several rest bars. Now comes rehearsal number 100ff. There is the famous "pastorale" c2-f2-f1-c2-f2-f1 etc., which can be played all Bb-side but fingering the top f2 with first valve or you do it also on the F-side, which might produce a smoother sound & smoother lip-legato. And finally the final solo at 107. Ooops I forgot the measure before 106 ending on pianissimo g#2. If the second & third valve slide are in the absolute position, nothing might happen, as the g# on the Bb-side, fingered 23, is just the same good positioned natural tone as the g2 on the F-side (here the same 12th harmonic on the F-sharp-horn). Now 107. Just relaxed play until the three measures rest. Make it easier for you, fingering the long held f2 with first valve on the Bb-side, to keep the sound at the right pitch (as I said earlier, first valve fingered f2 on the Bb-side means using the quite sharp 9th natural tone !). The final 109 solo requires a mixture of F-side & Bb-side. Start on the F-side, sneak to the Bb-side for the slur Efl - g, immideately back to F-side for the relaxed sound. Maybe the second horn could sneak in for the long held Efl. for a moment giving breathing time to you & sneaking in again for the written Bb just before the f2 entrance, providing another breathing & thus relaxing moment. Enter the f2 with first valve on Bb-side, return to the F-side with the bnext note (d2) & stay on on the F-side, shorten the long f1 for an eight to breath fast again before the d2 entrance still on the F-side (the d2 on the F-side is one of the most beautiful notes on the horn ever as it shines like a rising sun !), maybe returning to the Bb-side for the closing d1 & Bb, which is the most delicate note, as one is at the end of the piece & all concentration resolves, which can result in shaking. So the Bb-side might prevent it. The last entrance is on the Bb-side surely, an f2 with first valve to keep the pitch. 

Hope, you found this message interesting & helpful.