I am thoroughly convinced that wool is the way to go for layering. I have a single Smartwool longsleeved base layer that is versatile enough for the entire winter. It has a tall collar with a zipper to allow me to adjust for temp. I can wear this layer up till about 65 degrees. After mid 60's it get's a little warm though. Then I start looking like Fred when he wears a wool jersey on an 80 degree day. Not pretty.
Here is a review of this base-layer. This morning, since I needed to wash my Smartwool shirt after wearing it a few days, I used my old cool weather layering ensemble from last year; a Craft sleeveless baselayer, arm warmers and my jersey. It was amazing how much colder the Craft shirt/jersey was compared to the long sleeved wool base-layer with a jersey. So now I have all these Craft sleeveless under shirts that I'd rather trade for Smartwool lightweight/midweight Zip-Ts (the official name of this garment - if you want to do a search). Anyone want to buy some Craft under shirts?