One thing I learned last spring is that riding in freezing weather for a few hours is not the same as riding in freezing weather all night long. Continuous riding requires that your clothes be really good at removing moisture. Before it started sleeting the morning of the hc race I was already completely soaked in sweat. Even my gloves, which were inside glove liners were completely soaked. I was wearing gear that had always worked great for me before but obviously wasn't up to the challenge. Being completely soaked adds a fair amount of weight, not to mention discomfort. Sorry, I haven't figured out what to do about it yet...
 
If you are traveling for just a few hours at a time, layer clothes, starting with a base layer that wicks. I don't have to wear tights until it gets down below 40F and tights and a wicking base shirt and those plus one or two outer shirts/jerseys usually take me down to around 25F. I usually draw at line at riding below 20F as I have to get too bundled up to be comfortable handling the bike though I can go lower on the trike as that's like adding 10F to the temp with the fairing. When you start out, you should be about as cold as you can handle as you will likely warm up within about 5 minutes of pedaling.

As Kent says, staying dry is most important, as moisture and cold is a ride killer.  My general rule of thumb.
 
low 50's - maybe a thin head covering, single layer up top hand coverings.
low 40's - head covering, double layer on top, single layer on bottom, gloves, wool socks single layer.
low 30's -head covering, jacket, double layer on top, single layer on bottom, gloves and liners,  
            2 layers thin wool socks, or regular shoes.  
low 20's and below - everything above, plus another base layer and usually a baclava.
 
wind and water can make me drop down a notch despite temperature.  base layers are usually the cheap polypropolene affairs at Academy or such.  I like the Gecko Gear Ice tights, and wool socks, wool glove liners, cheap thinsulate glove shells.  As pointed out before gloves and socks have to be loose fitting to allow circulation, false help to wear multiple layers and have them be too tight.  My Gecko gear tights have held up over 8 seasons of winter commuting, and I probably need a new pair this year.
 
Take Care,
Nelson.
 
 

the only real thing I have to add to the conversation is WOOL.  Stays warm even when soaked with sweat.  I have a wool baselayer that's actually a rowing top and it's been the best piece I've ever owned.

Velomobile with electric heater.

Darrell,
 
Before you invest too much in stuff for your ears and neck, try a thin lycra balaclava. It won't look thick enough to keep anything warm, but you will be surprised at how well one does, and when it warms up enough to take it off, it won't take up much space in your bag.

Quote from Hull 697 on Aug 12^th, 2009, 12:39am:

 
I started out with a thick one too, and soon passed it on to Rose. I can't recall for certain where my thin one came from. It may be a Nashbar Polypro.

I really like the lycra hood one I believe I picked up at either dicks or cabellas in the hunting section.
Mine was that hunter orange color but here is a pic in white but you may get teased about being that good ninja from gi joe.

It is one of those 5 in 1 headgear things that does the  neck gator, hat, balaclava with nose covered, etc and really worked well in most.
It is very lightweight and packs very small.  
Here is me wearing it in the old says back when I did brevets. Hm those are sandals I'm wearing  

 
I find if I can keep the wind off I'll be fine. Anything that warms or keeps heat it too much is way too hot for me.