This comes up from time to time - and in response to a fellow member of our group I thought it would be helpful to share with those who might be struggling at times with the concept of what helps make a great pin up character. So briefly ...
In an effective pin up you're looking to try to capitalize on a couple of things: Body language and facial expression.
It's assumed the model is beautiful and attractive - but how do you project that allure to the viewer? Eye contact, while not required, often makes a big difference. Avoid mundane everyday poses and instead ramp up the body language that accentuates her beauty and form.
Lastly, the facial expression is something that can turn a flat, emotionless 3D character into someone that leaps off the page and makes the viewer sit up and notice. Sex appeal is largely the projection of the energy of that person, whether it be playful, flirtatious, smouldering, romantic, haughty etc. - this is mostly achieved through the face. It says to the viewer I'm beautiful, I see you, and you definitely see me!
As a help, I would suggest for the face - take a look at the "Close Ups & Portraitures" gallery to get a sense of what we mean about "facial personality". And also take a look at the Featured gallery or the Pin Up Examples gallery to get a sense of what we're looking for in terms of body language. Hopefully this makes sense, and more so is helpful.
Happy posting everyone!