When she really doesn't want sex, she might be persuaded to lie on her side and let you slide up and down between her buns (lots of baby oil needed). Feels really nice, especially if she is lying more or less straight. Let her steer you by hand or you'll slip out. If she wriggles or squeezes her cheeks it will do very nice things for you.

What's in it for her apart from a fuzzy generous feeling? Well, it won't take long to figure out that rubbing you against her star feels - well - very nice! And let you come while pressed against is even nicer. Don't ever try to force the issue, and you'll get there eventually.