A survival bag of some sort is a must, especially in winter. Traditional options are cheap and cheerful, but heavy. This version from Lifesystems is very light at 205g and packs in to a 10x7x7cm stuff sack. The reflective surface radiates heat making it a warmer than a standard bag too, and far more practical than a foil blanket (next to useless in the great outdoors). It feels flimsy, but is surprisingly tough.