Tried one of those silk liners for a few nights; took me several minutes each morning to get untangled. No apparent increase in warmth, and no apparent decrease in warmth when after a few nights of this nonsense I tossed the liner in the nearest trash can.

Like Glenn, I use my insulating clothing if the temp goes below what my sleeping bag can handle. I also use my non-breathable rain gear as a vapor barrier over my base layer when it's freezing or below. That really does add appreciable warmth and keeps my "insensible perspiration" out of the sleeping bag insulation. Any insulating clothing added goes outside the rain gear so it won't get damp from my body moisture.