I use my closed cell foam sleeping pad. I strap it to the bottom of my day-pack. I thought about cutting it in half, but the length is pretty nice when I want to sit with my legs out in front of me or even lay on it for a while. It weighs next to nothing and sure keeps the cold and snowy ground from taking away your body heat.
4 sections cut off a Thermarest Z-lite. Weighs 4 ounces and is great to sit and kneel on and put in the vestibule of the pack tent when you take off your shoes. The z-lite is 10 sections long in stock form and I made three glassing pass out of it. Actually if I bought one today I would cut it half for an extra length butt pad for the noon naps!
I have spent a lot of time sitting on the ground, crazy-creek chairs and sleeping pads. I just ordered on of these Helinox chairs and thinks it's definitely worth a little extra weight: