As a stock photographer, I need to direct models so I can capture a specific expression or body language to convey the concept I want. Models also need to know what they are doing if they want to be successful. Here are some useful online articles about posing for models and photographers. Different photographers work in different ways so I don’t necessarily agree with everything in the articles but they are a worthwhile starting point.
Posing 101 for Models and Photographers has some very good advice from a commercial photographer. The second half of the article has a long list of links to related topics.
Mary Duprie specializes in model photography. She has created a series of videos that demonstrate her methods of working with commercial and glamour models.