Be like an ogre/onion - have layers. You can throw on another pair of tights over your leg warmers and bibs for extra insulation. I have some fleece lined Adidas ones that are ��.
I also always have a hip pack on so I sneak in a lightweight rain/wind jacket just in case. You can also just stick one in a pocket or rolled up under your usual jacket too. Useful if the weather changes, or if you have to stop and get your outer layer wet.
In that same vein, nitrile gloves can turn an unbearable ride into a bearable one. They take up very little space and can be used on your feet or hands. They're especially useful if you have to stop and fix something on your bike.
And speaking of fixing things, make sure you're probably geared up to fix things. I wouldn't mess with patching a tube in the cold unless I had to. I'd just tag in a fresh tube to speed things along. Bring two tubes and a real pump just in case.
Battery pack handwarmer - charge your phone or computer, or stick it in your middle jersey pocket under a jacket for some extra toastyness.