A modular system is what you need. Base layers, insulation layers, and a shell. You shuffle them as dictated by weather in addition to a basic overnight survival gear bag that goes wherever you go, regardless of the type of pack you carry at any given time. For me that includes a disposable bivy bag, fire making, water purification tablets, and energy bars for food.
Specneeds mentioned a good lightwieght packable down vest. I carry one in a stuff sack in my pack, it's about the size of a softball. Worn with a good shell (pitzips are critical in a shell IMHO) you can survive overnight if need be.
Last edited by BeanMan; 10/28/14.
