My first priority is clothing adequate for conditions more severe than the worst I expect. First priority is a good shell parka (goretex or similar). I carry this even on short hikes when it looks to be warm and sunny! Probably shell pants as well, and plenty of insulation. I tend to get warm when I hike, even in cold weather, so I'm usually dressed more lightly than my buds, but carrying more clothes in my pack. I also have extra food, fire starting gear, etc.


On day hikes in summer, generally the only additional shelter item I carry is an Adventure Medical SOL Emergency Bivvy (the little lightweight $16 one). http://www.adventuremedicalkits.com/products.php?catname=Shelter&cat=53

Winter is a different story. When backcountry skiing, I swap the little SOL bivvy for the more robust SOL Thermal Bivvy. I also carry an ensolite pad. I also have a shovel (as part of my avalanche gear). In addition, stuffed in the bottom of my pack I have a puffy synthetic filled insulated jacket sized to fit over ALL my other clothing (this is in addtion to my normal goretex and fleece gear noted above). Also dry socks and dry glove liners (vacuum sealed).

NOTE: Thanks Izzy for the tip that Adventure Med is coming out with a breathable bivvy bag. I will have to check that out when they are available.