If you don't already have a turtleneck, go buy one. Now. Hell, even if you own a few, you should consider a one more -- maybe in super thin merino wool or cashmere (it'll take up less room in your suitcase).
I have recently fallen in love with the turtleneck. It's a fairly new relationship, inspired by Nora Ephron. It elongates my neck, keeps me warm and has a simple elegance to it. It's a perfect winter staple because you can wear it with anything.