As a general rule you should feel that the mouthpiece pressure is basically the same on the upper and lower lips. See The Jaw and the Lips . In the past it was felt strongly by some brass experts that only thin lipped people should play the trumpet and horn, and thicker lipped people were more suited for the trombone and tuba. There is perhaps some small element of truth to this, but successful players with all thicknesses of lips may be found on every brass instrument. I have rather heavy lips myself and do not feel that it has caused any significant problems in my playing.
Lateral pressure is mouthpiece pressure which forces the mouthpiece up, down, or to the side.
This is undesirable, not only because it cuts endurance, but it also forces the lip aperture out of its proper, natural shape. For hornists a good way to work on a lateral pressure problem is to practice standing, as it is more difficult to apply lateral pressure with the horn off the leg. While it is probably true that this basic embouchure set up is not unusual on the lower brasses, it is quite uncommon in the higher brasses, at least among fine professional players. The basic ideals are outlined above. Copyright John Q. Ericson.