One other thing I don't recall having seen in the myriad of good advice that has already been given is to be wary of compression with your down bag. Sometimes the weight of multiple layers in or on the bag will compromise the insulation of the loft. Select areas of compression can also cause cold spots. This occasionaly happens to me in the elbow areas in particular. I usually pull my bag out right when I get the tent out, and then fluff it again before I climb into it at night.