Keep producing heat: move, eat, drink.
Avoid standing or sitting still.
That's fine until you get lost or have a mechanical or whatever.
My circulation's adequate, so hands and feet aren't a particular problem, but I do a good line in getting the shakes and having great difficulty getting warm again (usually requires a hot shower or getting back on the bike and attacking a proper hill). I assume this is why audax controls are traditionally located just before a big climb
Does having a hot drink help you at all?
I'm not sure, but I'm not really very good at hot drinks, not being a fan of tea (understatement) and preferring the smell of coffee to the taste. I'll drink hot chocolate where available (if I'm clever, I'll bring a sachet or two with me), but I tend to wait until its temperature has dropped to the point that it doesn't leave my tongue numb for the next few hours, which is probably not hot enough to be of significant benefit. I don't understand how people can bear to drink steaming hot liquids.
What I tend to do on winter rides and cold night rides is carry plenty of extra layers (down jacket if it's properly cold) and put them on as soon as I stop, while I'm still warm. And make sure I do any bike faffing, refilling of water bottles etc first so I don't have to go back outside and hang around in the cold before setting off again. I'll check the temperature (bike computer with a thermometer is very useful) and take most of the layers off before setting off again, much to the confusion of everyone doing the opposite.
The tricky one is the Friday Night Ride to the Coast, and its regrouping stops of unknown duration. You might be there for a half a minute, or it might be ten, depending on what's going on at the back, and you won't be popular if you have to stop to remove layers and put them away when the call to set off again comes. Best method there seems to be to ride nearer the back of the group in cold conditions, so you can maintain a more even power output and spend less time hanging around.