Starting notes in the low register is much easier when we visualize the embouchure and aperture differently from high and middle range horn playing. Setting the embouchure is key. Ron Bishop, former tubist of The Cleveland Orchestra, once showed me a European tuba method book; on the cover was a photograph of a monkey! I asked him about it and he said that old time European tuba teachers believed a monkey's face was a good image for a tuba embouchure. I began experimenting with monkey face, visualizing it when starting notes in the low register. The low tones seemed to jump right out!
Thinking "monkey face" softens the corners of the mouth, helping them move toward the center. "Monkey face" also helps to increase the size of the oral cavity by lowering the jaw and produces the optimal aperture size and shape for starting low notes.
If horn students aren't specifically taught how to play in the low register, they sometimes come up with unusual ways to configure their lips and facial muscles to produce low notes. A frequent aberration is the "frown" or "trout" embouchure, in which the corners of the mouth curve downward. Players who use the trout embouchure are sort of on the right track, since it's easy to think that bending down those corners would make the oral cavity larger. Unfortunately, the jaw actually rises up when we bend the corners down.
Mimic the monkey's face on this page as you start low notes. You might be surprised at how much this little monkey can help!