Formation of the Lips:
One famous Horn teacher describes the ideal lip formation as a "puckered smile." The lips should be placed in their normal relaxed position and then slightly pursed (stiffen them without bunching up the chin). Too much pucker in the lip formation will produce a stuffy, flat tone quality. Too much smile will cause a thin, airy, sharp sound.
The mouthpiece should be well centered (left to right) with more upper lip in the mouthpiece than lower lip. Many of today's best Horn players place the rim of the mouthpiece within the pink portion of the lower lip.
Changing the size of the opening between the lips is a very effective way to change pitches. Focusing the muscles of the embouchure toward the center to create a smaller aperture will produce a higher pitch. A lower pitch will result if the aperture is enlarged through relaxation. This method of changing pitches is effective only if the muscles of the embouchure are free to work: use as little mouthpiece pressure as practical. Do not use excessive movement of the corners of the mouth to change pitches, as this type of movement would adversely affect tone.