A point that I have to make often in lessons with my students is that in order to expand your dynamic range, you have to go beyond the point where it sounds good. Get out of your comfort zone. I call it "ugly soft" and "ugly loud." If you don't push your limits, you won't improve. This is true for most things in life.
If we keep doing what we're doing, we'll keep having what we're having.
Related to this topic is the "Someone Might Be Listening Syndrome," which causes students to play things they're good at over and over in the practice room, while ignoring the passages or aspects of their playing that really need work. I don't know who said it first, but "if you always sound good in the practice room, then you're practicing the wrong things." Maybe it was Tuckwell. (He's kind of the Mark Twain or the Yogi Berra of the horn world; most clever quotes are attributed to him!)
Let me be clear: I'm not advocating playing with an ugly sound in performance. This is merely a practice tool. The goal is not to achieve an ugly sound! The goal is to be able to play louder and softer with a beautiful sound. In order to achieve this, you have to spend time practicing in the "Ugly Zone."
The traditional long tones with crescendo/diminuendo are the #1 best way to expand your dynamic control, because they take you gradually into and out of the "Ugly Zone," and it's easy to track your progress as the "Ugly Zone" gets pushed farther and farther in either direction. I know what some of you are thinking, but if you're focused on everything you should be focused on while practicing long tones, they are NOT boring!
Besides these long tones, another great place to practice these "extreme dynamics" is in etudes. I'm a big fan of Kopprasch (my students don't always share my enthusiasm), and I always write something at the top of the first page in my students' books that my first teacher wrote in mine:
Too often, students are afraid of getting outside their comfort zone, and therefore they don't improve as fast as they might. There is always an adjustment period, whenever we're trying to build new skills or expand existing ones. Think of a baby learning to walk. It looks pretty awkward when he/she tries taking their first steps, and there will be stumbles and falls. But, they can't go on crawling forever!
Don't be afraid of sounding bad in practice. Be more afraid of sounding bad in performance! Attack your weaknesses!