I use salt sometimes, but I've recently been keeping a bottle in my back pocket. Not a normal water bottle but a small nalgene. I drink from the water bottle in the cage for the first 30-60 minutes, then when it freezes up I'll pull out the nalgene and drink that. I will then pull over at some point, open the top of the freezing water bottle, dump whatever wants to come out into the nalgene, and put it back in my pocket where the frosty turns back to cold water.
It nets you almost 2 bottles. On the coldest days the water bottle freezes up fast, so you'll be lucky to get 30 out of it. Just drink an extra 2 cups of water before you leave the house. If you're going for an epic, then stop somewhere and refill. If you're not, then you'll be all set.
The nature of winter riding tends to be lower intensity, so the factoid that you dehydrate faster in the winter is an incomplete truth. You go slower, work less intensely, and thus sweat less. If needed, I can go 3 hours on 1 bottle in the winter and be fine. In the summer I'd be hallucinating if I tried to do that.