To help keep drink tubes and bite valves from freezing up, sip often to maintain flow, then blow any residual water back into the reservoir after you’ve finished drinking. It helps to squeeze the bite valve momentarily to remove any last drops.
It helps to keep as much of the drinking tube covered as possible, such as routing it inside a pack, strap, or jacket. You can also start off the day with warm or hot water in the reservoir, packing any extra clothing layers around it.