Unlike most who have wieghed in, I am a complete amateur and pretty green. But one trick that has worked for me (although I realize it has many limitations) is to use the model's hands. Get her to rest her face on them, or pull a collar of a garment around her. I don't know if this will work for what you have envisioned, but I have had some pretty good results with this, especially with self-conscious people and people who are... uhm... on the heavier side.

Peter.