Actually, I recommend something a bit different: I recommend you start at pppp, so soft you can hardly even start the note, build up to ffff, so loud it wants to break up, then back down to pppp, so soft it starts trying to drop out. Part of the purpose of long tones is to increase your dynamic range while maintaining a good tone and pitch. So if you go from mp to mf, never exceeding the range where you can keep everything nicely under control, you're not extending your capability. If you work at fff, and it wants to break up, and you keep practicing, after a while you'll be able to keep control at fff, and it won't start breaking up till ffff. And you keep at it, and you'll be fine at ffff, and it won't be till fffff that it starts getting wild. And the same thing down at the soft end of things.
You might poop out and have to do, especially on low notes, just the pppp to ffff part before you run out of air. Then take a breath and go ffff to pppp. That's fine. You might have the thing split and blow apart, and have to back up and get another run at it. That's fine, too. On the soft end, it'll drop out on you. Might have to step back, start back at p, and diminish again. Might do this three or four times. Keep at it.
Then, of course, my other constant advice, is to whenever possible do this OUTDOORS, away from reflective surfaces. YOu may think you're blowing the doors off that small reflective practice room, but get out there in the great outdoors and listen to your sound evaporate. That's what the old guys used to do. You know, no-counts like Sonny Rollins. You want a rich, husky, compelling sound that evokes the wind on the prairies, the thunder on the plains, the crashing of surf on the craggy shores. Don't be satisfied with a practice-room sound!
