I started out a few years ago using a silk sleeping bag liner, expecting it to add warmth to my 30* bag and to keep it clean. I had two problems with the liner: First, I would be so tangled up in it by morning that it would take up to five minutes to get unwound and out of my sleeping bag. Second, it added only a degree or two, at most, for warmth. The 9* additional that the makers of the silk liners claim is strictly hype, IMHO (and I'm not the only one with that opinion). It might be fine for those who don't toss and turn at night, but only to keep the bag clean.
I now sleep in my base layer. Since it is worn either at night or under my outer clothing, it stays pretty clean. I wash face, neck, ears and hands in the evening, and generally wear a hat while sleeping. This routine keeps my bag reasonably clean. I'm giving up wearing my base layer top as my hiking shirt because it gets so grubby–dust, sweat, the inevitable food spills–and I really don't want all that in my sleeping bag. That's particularly true of the food spills! I know the sleeping bag has to be washed eventually, but I don't want to do it too often.