A lot of the time we have a 20 to 45 km ride to get to the mountain riding, so if it's -20 to - 25 c it can be very cold. I have a thin mitt style shell I slip over my gloves to cut the wind, a bit awkward but you get used to it, I then pull them off once we get to the mountain riding.
As far as the gloves go I've tried alot, usually have several pairs along and change them throughout the day as they get wet. Right now I have a high tech pair of ski gloves with are the warmest I've tried, they have a pile type lining in them which wicks the moisture away from you skin.
I always try to keep a fresh pair for the ride out especially if it's a long one.