The lightest and most reliable solution is to invest in a warmer bag. Anything else: adding clothing, adding liners, etc., will also add weight. That being said, I'm not married to being ultra lite, so I have a fleece liner (this one: http://amzn.to/2aGTTga) that I bring when I'm pretty sure my bag is not going to cut it.

What sort of pad do you use? Sometimes if my bag is right on the cusp of comfortable temps,I bring along a closed cell pad (I use the Ridge Rest http://amzn.to/2au1WNX) and put my inflatable on top of that.

I am also a big fan of using a handwarmer at the bottom of my bag at night in cold temps. It's amazing how much that helps without adding much weight. I can also then throw it in my boots in the morning while I'm eating breakfast to give them a little warmth before I put them on.