When posing a subject for a portrait you have to think about more than just body position and facial expression, you have to study every part of their body. Digital Photography School has a breakdown of thing to think about, body part by body part. Here's a glimpse at what they suggest:
Womens' fingers should be long and elegant.
Feet at different angles or heights (on a step, chair etc) will give better “balance” to depth.
Elbows bent express a comfortable casualness.
4. Head / Chin
Head tipped back to the “high” shoulder will feel fun and flirty
Head down toward the “low” shoulder can express power or position (especially when shooting up at the subject).
Legs spread hip width apart while standing will give an air of strength.
Should be on different "planes" (i.e. one slightly higher or lower than the other)
One rule: If it bends, bend it. This goes for elbows, knees, wrists, etc.
Women who stand with hips tilted forward will appear more slender.