one other way would be to treat the woman and couch separately. then place each on their own layer in Photoshop. you can then lower her "into" the surface of the couch (as anything in the real-world bearing any amount of weight would do...) you can use various tols to distort the surface of the couch to make folds and indentations around her...

it's always easier if you can freely move elements around to develop a perceived reality.

scott