Apologies to Snow White and the Bears for the mixed metaphor…
After several “fails” in overdressing, the combination I’ve been wearing outside the last few runs in the 30s worked well in the low 20s. However, I admit it probably still could have been a little lighter to minimize moisture buildup. The vest is a great tool, but hard to walk out the door in when it’s so cold.
The rundown:
1) unmentionables
2) L/S compression shirt
3) running tights
4) shorts
5) L/S fleece (here’s what I’d swap out for a vest – but a S/S shirt under this might have worked better)
6) hat
7) lightweight gloves
Low wind makes this calculation so much easier to figure out right. Wind goofs everything up – unless you can keep it out without holding in too much heat. RW has a pretty good web app that can get you in the ballpark depending on the weather conditions.
cretanrunning 
I keep notes on my runs and what I wear. At least… I used to. Now I have a chart that I keep in my closet. This year, I adjusted most things so I run a little lighter. I also added a breakpoint for light gloves vs. heavy gloves (at about freezing). I put on a compression LSS at about 55 and add gloves. Then at 40 I wear tights. Then at 30, it’s heavy gloves, running shirt and pants over the LSS and tights with baclava! The baclava rocks. Then I’m pretty much good down to 20. At that point, I have a cheap Champion shell that doesn’t breathe. It’s perfect for the super cold.
For wind, I pretty much take a look at the windchill and decide if I want to adjust. The fun (challenging) thing I’ve noticed is that the “blackberry” temperature runs about 5 degrees colder than my bathroom thermometer (most days). And my chart is based on *my* thermometer. I’ve decided that the blackberry must take the temperature down by Mill Creek. Because if it’s still it can be really cold down there. And my house is on one of the highest points around. I know, because I’m always running up hill on the “backs”.
I run early in the morning, so the sun is not a factor. But a clear, still day can make a big difference on what you need to wear to be comfortable.