If I don't want to add bulk, I usually use a layer of flannel in between. Well washed and dried to preshrink it of course.
But, I'm almost converted to another product.
Micro fleece.
This stuff is almost indestructable, super soft, and not bulky. I'm not sure if it's the same stuff as the minky that people talk about.
It is stretchy though. You have to baste it down well before adding the top layer.
I just finished a bargello using this for batting and it turned out fantastic.
And - best of all, it's cheap in comparison to other options. I just buy a blanket at WM and use that.