Since batteries (for cameras, cell phones, etc) will lose charge in cold weather, what can I do to prevent loss of charge, or at least extend the battery life as long as possible?
The cold can cause the electrochemical processes in the battery to slow down, eventually giving you less than it's usual voltage. (Why don't batteries work as well in cold environments?)
If I need to use anything in the cold, I always keep it next to my body or in my sleeping bag at night. Try not to roll over them in your sleep!
If you can, use lithium batteries in your devices they will last longer and are less susceptible to the cold. My old gopro wouldn't shoot video in the snow unless I had lithium batteries in it. The regular NiCd or NiMh batteries could only do the less power-intensive photos.
Photographers in the Antarctic store camera batteries in their armpits to keep some decent heat in them. Amusingly, when the batteries in the camera die, that is also the best place to put them. My dad used to test cameras in the Antarctic (in addition to his day job) and said that was the worst bit at 50 degrees below freezing - popping a cold battery under your arm!
One approach might be, where possible, to use Eneloops. Far from losing their charge rapidly in the cold, they actually retain their charge far better when they're kept cold (and in general retain it far better than normal rechargeables.) I've got 8 AA ones I use in various pieces of gear (torch, GPS) and they've been great.
edit: James Dunn
Eneloops have a "much" longer storage time than NiCd or standard NiMH (lose about 5%/day). They are reported to lose 10% of their charge in 7 months. They have a large current potential to better provide quick cycling of speedlites (heavy loads).
If charged in a freezer they will load about 25% more total power but the charge time is slower/longer. But for fast charging, charging in the freezer is still better to keep the temperatures under control. If kept frozen I have no idea how long they would hold their charge.
However, if discharged below 60% of full charge voltage, this permanently damages the cells and they will not charge to full capacity ever again. Discharge is linear from about 95% down to 70% of discharge voltage, but from 70% to 60% the time is very short under the same load. So it seems that it is better to keep them topped off, but prevent overheating during charging.
Put them close to your body several hours before using them.
For seldom used, small load battery use, I am sure there are more durable batteries. I have lithium ion batteries in a security system that have been running wireless contact sensors for more than 5 years.