Step 4: Dress Warm
However, I must maintain my position, due in no small part to the excellent links provided by the thermally impressive gmoon, particularly this one.
I recommend a synthetic fleece vest or wool sweater and a knit cap.
The vest is light, dries quickly and is quite warm.
The sweater is even warmer and in my experience slightly more compact than fleece for the same amount of warming.
Both remain fairly insulative even when damp, unlike cotton sweatshirts which are literally worse than nothing in wet situations. Though I prefer the wool sweater overall, synthetic fleece is nice in that it absorbs almost no moisture, whereas wool takes on the approximate density of depleted uranium when it comes in contact with water and is very reluctant to dry. One other noted difference between the two is that wool is all but fireproof, whereas any synthetic cloth is prone to burn quite easily and melts while burning as well, adhering to skin like overpriced napalm. While this is usually a minimal hazard, small sparks from a campfire are notorious for burning little holes in synthetic clothing.
The hat is amazingly warm, and while I usually use wool knit, synthetic is also excellent.
I should also add that the inestimable zwild1 reminded me of the importance of the bag liner. This is a cloth (I use synthetic fleece) liner which goes inside your sleeping bag. It can improve the rating significantly. Good suggestion!