60 degrees +: tank top or singlet and shorts (consider a cap or hat to keep sun off head or hair band to keep hair out of eyes)
50-59 degrees: short-sleeved and shorts (consider a cap or hat)
40-49 degrees: long-sleeved top, shorts or tights or wind pants, socks or mittens or gloves (consider a cap or hat)
30-39 degrees: long-sleeved top and a long or short-sleeved top layer, tights & shorts, socks or mittens or gloves, and hat over ears
20-29 degrees: Polypro top or thick long-sleeved top, another long or short-sleeved top layer, tights and shorts, mittens or gloves, and hat over ears.
10-19 degrees: Polypro top and thick long-sleeved top, tights and shorts, wind suit (top and pants), thick mittens, thick hat over ears.
0-9 degrees: Two Polypro tops, thick tights and shorts, (and thick underwear or supporter for men), Gore-Tex or similar thickness warm-up, gloves and thick mittens, ski mask and hat over ears, and Vaseline covering any exposed skin.
minus 15 to minus 1: Two thick Polypro tops, tights and thick Polypro tights and thick underwear (and supporter for men), thick warm-up, gloves, thick (arctic) ski mask, and thick hat over ears, Vaseline covering any exposed skin, thicker socks on feet and other measures for feet, as needed.
(Thanks to www.jeffgalloway.com)
Notes: These are only recommendations; use the combination of layers which works best for you.
Wear tops, shorts, tights, socks, gloves, mittens, hats/beanies made of material that wicks away moisture from your skin. Keeps you dry and you are less likely to chafe.
A Change of Clothes: For training sessions away from home (trail, track, etc.) and for races bring along a set of warm and dry clothes to slip into immediately after training or competing to keep from getting chilled by wet training clothes (yes, even after warm weather training).
From "Runner's World," click for a fun and easy tool to help you choose what to wear: What To Wear