The trick is to blow the water back up the tube and into the reservoir right after you take a drink.
This will keep your tube and bite valve from freezing.
This works well even at below freezing temperatures when skiing.
bicycles.stackexchange.com/users/5371/Wadelp
I see snowboarders with an insulating cover over the tube.
If that doesn't provide enough insulation, I've worn my pack under my jacket leaving the entire pack, tube and bite valve covered and insulated.
Here is a 3 foot Hydration Pack Insulated Drink Tube Cover on amazon for $7 US
bicycles.stackexchange.com/users/2953/Glenn Gervais
Keep the bladder as close to your skin as possible.
This will help it utilize the same heat and insulation you are using.
Run the tube under your shoulder.
The area over your shoulder is more exposed to wind and will generally freeze faster.
After taking a drink, blow back into the tube to clear it.
If there is no water in the tube, there is nothing to freeze.
(Optional) Insulation.
Many companies make neoprene or other styles of insulated covers for their drinking tubes.
While these can help maintain some warmth, they can also make finding and breaking up ice chunks difficult.
You don't need to completely clear a tube to drink from it.
You just need to break the ice up some
so the warmer water (from your back) can run through it and accelerate the thaw process.
(Optional) Power.
There is at least one company that makes an on demand heating system|water bladder.
It uses a battery pack and heating element throughout the whole tube to melt the tube even if it is completely frozen.
bicycles.stackexchange.com/users/13710/Deleted User
The tube has a lot of surface area and not a lot of volume, so it's going to lose heat quickly compared to the reservoir.
In addition to insulating the tube mentioned by Glenn Gervais you can start with hot water in the reservoir and frequently drink a little bit to keep reheating the tube.
This Nordic skiing article discusses this technique in more detail.
You could also try a Nordic skiing water bottle holder like the one in this (5 part!)
article .
I've noticed while skiing that a horizontal bottle seems stay ice free at the valve longer.
In a vertical holder, turning the bottle upside down also helps.
I've never tried this, but you might also try to keep the water in the reservoir warm by attaching a chemical hand/foot warmer to the outside of the bag.
Test it first to check that it won't damage the plastic bag, though I wouldn't expect something made to keep near your skin would get hot enough.
Don't put it in the bag though.
If it leaks it's probably dangerous to your health.
bicycles.stackexchange.com/users/510/Wayne Johnston
Here's a summary of some the options I've found: Manual:
Blow back the water back in to your bladder to prevent freezing, this can cause your hydration reservoir to bulge though.
I've also heard this can introduce bacteria to the bladder, making it get funky faster.
This is less effective when you have a smaller amount of water in your bladder.
Use a compact reservoir and keep it under your jacket.
Run the tube under your jacket or down your arm under the jacket.
Keep it from being exposed when not in use.
Insulation: Neoprene insulation for your tube.
Bag makers have their own more expansive insulated systems than just a tube cover.
Osprey insulated delivery system, which has a zip up cover for your valve.
Camelback has the Antidote Thermal Control Kit which includes a cap cover for the bite valve and neoprene tube cover.
Salomon makes a fully insluated hydration tube and bladder with bite valve cover for nordic skiing.
Use foam packing material and tape to make an insulated sleeve for your bladder, to slow the cooling of water in the bladder itself.
Heat Managment:
Fill your bladder with warm or room temp water, so it will take longer to cool down.
Add hand warmers to the pocket in your hydration pack where your bladder lives.
May want to put this in a sock to prevent direct contact with the bladder.
This will keep the water around it warm, and prevent it from getting as cold so long as you keep drinking.
Chemical: Add a small amount of vodka to your water supply to prevent freezing by lowering the freezing point of the water.
I'm not quite sure of the appropriate amount.
Add electrolyte tablets to water, which should in theory lower the freezing point of water a bit.
Making it take longer for your water to freeze up.
Do not exceed the amount suggested by manufacturer of electrolyte tabs.
Not sure to what degree the freezing point would be lowered.
bicycles.stackexchange.com/users/2997/Benzo
I use rather warm water to start with by filling approx 1/3 of the bag with cold water and then the rest from the hot water dispenser in the lodge.
Going forward
it’s
mostly a matter of remembering to drink enough regularly enough to replenish the warmth into the tube.
This temp change is obvious as you drink.
I’ve found that each chairlift ride seems to be perfect.
bicycles.stackexchange.com/users/40840/David Rush
Putting a little glycerol (aka "glycerin" or "glycerine") in the water will help.
Glycerol is edible, sweet to the taste.
It will do double duty by giving you a few extra calories, and lowering the freezing point of your solution because it acts as an antifreeze.
Because it's also bacteriostatic, unlike sucrose/glucose, it also shouldn't encourage microbial growth in the hydration system.
Unfortunately, sports drinks probably shouldn't contain more than about 8% carbohydrate , and an 8% glycerol solution will only give you about 1-1.5˚C extra headroom .
bicycles.stackexchange.com/users/7263/sampablokuper
Having a quick release in the hose can help a lot.
Generally, just the valve end freezes and the quick release allows you to get a drink.
Click the system back together and stick the frozen end in your jacket.
Generally it thaws in a few minutes.
The insulated hoses don't do all that much.
By far the best solution is the packs in which the hose is entirely enclosed in the pack strap.
Backcountry Access Stash packs are a example.
bicycles.stackexchange.com/users/8445/Fred the Magic Wonder Dog
Some hydration packs (such as Source Hydration Systems ) have a wider diameter than others and/or an insulated tube cover.
Both of those can help keep water from freezing in the tube.
Disclaimer: I am associated with Source Hydration Systems.
bicycles.stackexchange.com/users/8437/Peter Sam
I like to ride with a hydration pack when I mountain bike in the winter since all sorts of nasty stuff gets tossed up at my bottles.  However, I've had issues with the tube or valve getting clogged due to the water inside freezing during rides in the low 20s Fahrenheit (roughly -5°C). It's really hard to deal with this issue while you're out in the woods.  Is there anything I can do to prevent this from freezing so quickly?
How to keep my hydration pack tubes and valve from freezing?
bicycles.stackexchange.com
<winter><hydration>
bicycles.stackexchange.com/questions/14740
bicycles.stackexchange.com/users/2997/Benzo
The trickto preventing water from freezing in winter is to blow the water back up the tube and into the reservoir right after taking a drink.  Many companies make neoprene or other styles of insulated covers for their drinking tubes.  In fact, Salomon makes a fully insluated hydration tube and bladder with bite valve cover for nordic skiing.  If that doesn't provide enough insulation, wearing the pack under your jacket and leaving the entire pack, tube and bite valve covered and insulated works.  The bladder can be filled with warm or room temp water, so it will take longer to cool down. Moreover, a small amount of vodka, electrolyte tablets or glycerol can be added to your water supply to prevent freezing by lowering the freezing point of the water.  The tube is more likely to freeze than the bladder due to the surface area, so drink regularly enough that the water does not freeze in the tube, and blow the water back into the bladder after taking a sip. Some products insulate the tube, or you can cover it or run it close to your body to keep it warm. The bladder can also be insulated or filled with warm water to prevent freezing. This warm water will also help to keep the water in the tube from freezing. Hand warmers can be placed next to the bladder to keep it warm. Adding something like electrolyte tablets or glycerol to the water can lower the freezing point.
The tube is more likely to freeze than the bladder due to the surface area, so rink regularly enough that the water does not freeze in the tube, and blow the water back into the bladder after taking a sip. Some products insulate the tube, or you can cover it or run it close to your body to keep it warm. The bladder can also be insulated or filled with warm water to prevent freezing. This warm water will also help to keep the water in the tube from freezing. Hand warmers can be placed next to the bladder to keep it warm. Adding something like electrolyte tablets or glycerol to the water can lower the freezing point.