Here's roughly how I dress:

> 50 degrees: t-shirt, shorts and socks.
35 - 49 degrees: throw on a sweatshirt, and full-fingered cycling gloves.
20 - 34 degrees: switch to long pants, calf-length socks and add a headband to cover my ears.
0 - 19 degrees: replace the sweatshirt with a wind breaker. Replace the headband with a balaclava. Replace the cycling gloves with snowmobile gloves. Switch to boots if there's snow on the ground.
< 0 degrees: add thermal underwear and socks as a base layer. Put the headband back on but pull it down for additional neck protection.

Oh, and those temps are Farenheit.