“John, what do you advise if you feel you are using the aperture corners properly but the aperture closes off? I suppose there’s only so far you can pull the drawstring before the bag closes up completely.”
By contracting the corners inwards towards the center of the mouthpiece / the air column to reduce the aperture (as opposed to pulling the corners backwards and stretching the lips out / clamping down top to bottom), this makes it easier to reduce the size of the aperture and is less limited–we can always drop our jaw down to open things up if we have the sides of the aperture pulled in tight. Whereas if we’re pulling the corners back and stretching out our lips, we are very limited–we can only go so far and we thin out our lips / have reduced endurance, etc.
Others have touched on the bottom lip a bit–Greg mentioned the “milk spout” concept, etc.
I try to keep things centered and try to employ a “less is more” concept to things–when I’m successful at that, I find things feel and sound the best for me. I experiment with slight differences in motions in my lips and tongue as I contract things as I ascend to get the best feel / sound / ease, etc.