they don't use expressions much in poser galleries (as noted above) because:

• they're hard to do right
• new poser users are mostly interested in posting a large-boobed girl in order to get congratulatory comments, and AFATK nobody looks at the face
• in the absence of affect, emotions and facial expressions have no meaning to them

however, one way to do it is to look in the mirror when trying to do facial expressions, or rotoscope it over a photo of a person with the desired facial expression