A lot of heat escapes through your feet and head, so double up on socks or have a very, very good warm pair that you use only for sleeping. For the head I use a fleece wrap and wrap it around my head and face as I see fit. I found if I could keep my feet and head really warm the quality or warmth of whatever I was sleeping with wasn't as important.
The other trick I found was keeping your stomach warm with an extra covering as well as insulating yourself from the ground.
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