In my travels this past week, I struck up a conversation with a head mechanic at one of the bike shops I visited. This gentleman does very long rides, even in the winter. It is not uncommon to ride 4 hours in below freezing weather. He shared a few secrets on how he stayed warm. Most of the things I had heard of, but one thing he mentioned, I thought, was ingenious and I had never heard before.
He buys underpads from Walgreens, cuts them in half and then puts this pad against his chest, underneath his base layer. The layer provides 2 functions: It first wicks sweat to keep the rider dry, especially when the rider is going with the wind. Next, it provides addional insulation and wind protection. For him, after 2 hours, the pad is wet and he needs to change it. So if you are going for a long ride, make sure you bring the other half with you in a ziploc bag.
I look forward to trying this trick.