[ivandiadser]

--What notable Buddhist teachers or traditions have specifically recommended smiling, or discussed it in detail?

Hi B,

I have remembered where I read about smiling in a meditation process.

It is taught by the Venerable U. Vimalaramsi in his essay The Anapanasati Sutta where he proposes a simple technique for the practice of mindfulness:

The “6R’s” are steps which evolve into one fluid
motion becoming a new wholesome habitual tendency that
relieves any dis-ease in mind and body. This cycle begins when MINDFULNESS recollects the “6R’s” which are:
RECOGNIZE
RELEASE
RELAX
RE-SMILE
RETURN
REPEAT
Development of mindfulness (one’s observation
power) recollects each step of the practice cycle. Once one
understands what the purpose of mindfulness is, keeping it
going all the time is no longer problematic and makes the
meditation easier to understand, plus it is much more fun to
practice. It becomes a part of happy living and this brings up
a smile. Remembering and recollection leads to having a
wholesome up-lifted mind.

p. 39-40
where the "smile" thing goes like this:

Learning to smile with mind and raising slightly the corners
of the mouth helps mind to be observant, alert and agile.
Getting serious, tensing up or frowning causes mind to
become heavy and one’s mindfulness becomes dull and
slow. One’s insights become more difficult to see, thus
slowing down one’s understanding of Dhamma.

p. 42