A wind blocking outer layer is really key. All the wool in the world won't keep you warm if the wind goes right through it to your skin. The nice thing about cycling is that it's so directional. You can have wind-blocking panels on your front, and leave the back open to vent sweat.

Plastic bags between your socks and shoes help a LOT (and really show how much of it is about the wind). If you normally wear thin socks during the summer, your shoes could be too small for you to wear thicker socks. Compressing your feet and toes even slightly will reduce circulation and make them colder than they'd be in lightweight socks.

Make sure whatever you wear around your neck is long enough that wind doesn't go down into your jersey.