My layers--summer northern Rockies (where it can snow in midsummer and nights are often below freezing) or 3-season Pacific NW Cascades:

Sports bra and full panties (I hate low-rise women's panties because the waist is the same place as the pack's hip belt).

Capilene 2 base layer. The top (a long sleeve zip-T) is my hiking shirt. The bottom half travels in my sleeping bag dry bag unless it's really cold; it's worn at night and in the early morning.

Lightweight merino wool T-shirt: an extra layer, either for more warmth (instead of a vest) or instead of the Capilene 2 top at night if the latter is wet or filthy. I get arthritis in the shoulders so a vest won't do the job, and the short sleeve T is lighter.

Wind shirt (lightweight, under 3 oz.). It's great for protection from biting flies as well as drizzle or wind.

Montbell UL Thermawrap jacket, my main insulating layer. I wear it in camp and, in cold weather, during rest stops. I would have to be really, really cold for me to wear it while hiking!

Rain jacket and pants, non-breathable. I wear when it's cold; in warm rain I either wear the wind shirt or just get wet. When it's below freezing I can wear them in the sleeping bag (over the base layer) as a vapor barrier.

Polypropylene fleece balaclava ("When your feet are cold, put on a hat.")

Polypropylene fleece gloves

Mountain Laurel Designs rain mitts to keep the fleece gloves dry when hiking in cold wet weather.

I carry an extra set of hiking socks and rinse my socks nightly. I also carry 200-weight fleece sleeping socks, my luxury item--they're soft and cuddly and my feet sigh with pleasure when I put them on!

EDIT: I find several thin layers far more adjustable to the actual temperature than one thick one. Even in the coldest weather, you don't want to be perspiring while you move (which wets your clothing) or, conversely, getting chilled.