These Smartwool liner gloves are a must. By themselves, they're good for me down into the high 20's on a bike. Plus, you can do things that require some dexterity. The Smartwool gloves are perfect for when you want add some warmth to heavier gloves or these mitten gloves. And if the big ones get too hot, the liner gloves will keep the chill off your hands.
Warm arms are important too. I use these Pearl iZumi arm warmers. They're nice because you can pull them off, when you get to your destination or you can bring them with you just in case you need some extra layers on the ride home.