Promoting proper facial growth in adults and children;
a) Chew with mouth closed
b) Sneeze with mouth closed
c) Yawn with mouth closed
d) Breathe with mouth closed
e) Sleep with mouth closed
f) Avoid foods that rot teeth; chew hard foods
g) Talk with your hands; become proficient with something involving your hands (piano, juggling etc)
h) Sing; different tones and pitches
i) Be bi lingual especially in a forceful sounding language like german or mandarin

Yawning and sneezing with my mouth closed has been very effective for me; when i yawn with my mouth closed i use my lips and neck and jaw muscles to suck air through my nose; it seams to me that if you cant yawn through your nose then then your nose has insufficient capacity to draw in enough volume. I dont think we are ment to yawn with our mouths open; case in point you can dislocate your jaw with a big yawn and yawn with mouth open makes you tired whilst yoawing with your mouth closed tends to increase alertness.

Sociological factors;
Learn a language that is abrupt such as German or Mandarin. I suspect the way one talks has an impact on facial features; I am Australia i notice that Australians accent is more conducive to bad facial features; for example when i talk i tend to not to use my lips but hang my jaw low and talk with a bit of a husky force; this a learned cultural thing.

Sing; appropriating a range of tones and degrees of loudness cannot hurt

Talk with your hands; there is a connection, especially for males between the ability to learn and the use of hands.