Here is an exercise, which is certainly not new, but perhaps deserves reconsideration. Although I usually incorporate long tones in my daily routine, I usually do so to build endurance and range, and I usually hold the notes at a steady mezzo dynamic level for about 5-6 seconds. I recently gave some thought about the control that is necessary to hold a very long tone with a smooth crescendo - decrescendo.
First, this exercise requires very good control of a steady air stream in order to execute a smooth dynamic over such a long time span. If one is asked to draw a crescendo on the chalkboard 3 inches high and 6 inches long, then asked to draw another crescendo 3 in. high and 6 feet long, it will obviously be much more difficult to draw smoothly diverging lines in the 6 ft. example. For the same reason, a crescendo-decrescendo lasting 30 seconds will require much greater control than short dynamic changes. The control gained through this exercise will directly benefit the control of dynamic expression in long lyrical passages.
Secondly, this exercise requires a control of embouchure changes, which are necessary, as greater or lesser amounts of air pass through the aperture. Obviously, the aperture opening needs to change size to accommodate the different volume of air that is used in loud vs. soft passages. As we increase the air throughout a crescendo, the aperture size needs to increase correspondingly, otherwise the tone quality becomes constricted. Conversely, as the volume of air decreases in a decrescendo, the aperture needs to become smaller, otherwise the tone quality becomes unfocused and sounds airy. This exercise will aid the player in gaining control of the aperture changes necessary to match the varying volume of air, to attain maximum resonance of tone quality at all dynamic levels.
This simple exercise takes only a couple of minutes, and could prove to be a very useful addition to one's daily routine.