I'm currently reading a book called Coming of age in the Milky Way that explains some of it (it's an ancient book, but fun 'Sunday supplement' kind of Science..;)It does explain all that..at least up to the part I'm at..;)
I dunno, most people sitting on furniture just leave a general impression (unless you're talking about the old beanbag chairs, etc), and often, depending on the resiliency of the cushions, fabric, etc., tend to smooth out as you get out of them.
Unless you're going to be doing a closeup of the affected area, a general depression (shaped like, well, sorta, you know..;) would do. If noone's there any more, a little hollow in the center of the cushions would probably do you. If someone's sitting there, you can just move the figure slightly into the cushions, and get the same general effect. Good luck.

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)