Simple solution, carry extra batteries in your pocket, or a specially designed carrier.
Unfortunately, there are problems with both of these methods.
If you carry batteries loose in your pocket, you run the risk of loose change or other pocket items accidentally discharging them, some times violently.
Both Surefire and Batuca make fantastic spare battery carriers. Both of them are great and perfect for carrying in or on your pack (see County Comm, T.A.D. Gear, or Maxpedition for outstanding belt mounted/PALS mounted pouches for the Batuca cases.) However, these carriers are a bit to bulky for every day pocket carry.
What follows is a solution I developed after being inspired by Tommi Potx and his instructable Several of 1000 Uses for Old Bicycle Tubes. Much thanks to him.
Because of the low cost and short assembly time, these are meant to be disposable.
Cost: <$5 (excluding batteries)
Assembly time: <5 minutes.
Step 1: Stuff Needed
For this instructable we'll be using 2 Surefire lithium SF123A 3 volt cells. It is also easy to do this with 1 AA, 2 AA, 4 AA, 2 AAA, 4 AAA, or any other combination of cells. Only use the same type of cells, don't mix AA's and 123's) Also, don't mix old and new cells.
2. A length of bicycle inner tube.
The length, diameter, and type of type of tube used is dependent on the size and number of batteries being enclosed. For the SF123A's well be using 4" (10cm) of road tube. If you're packing the cells end to end you want the tube to be long enough to cover the cells with about 3/4" (2cm) of tube stick off on either end (see picture 2). AA, and 123A's can be packed end-to-end comfortably in road tubing. AAA fit into road tubing side-by-side quite nicely. You can also fit 4 AA side-by-side in mountain tubes (see picture 3)
Tubes can be found new at bike shops for under 5 dollar, and can be found free in the dumpsters behind said shops. It doesn't matter if they have holes in them or not, you'll be cutting them up.
Make sure you are using tubes that do not (nor have ever had) any kind of sealant in them (Slime, Stan's...) They need to be clean and dry on the inside.
3. 2 Zip ties.
The small thin ones work quite well in this application.
4. A measuring tool.
To measure out the length of tube
5. A cutting tool.
To cut the tubing.
Step 2: Load 'em Up.
2. Slide the second cell in negative terminal first. You want the cells to be negative to negative. If you're using cells that have a base rim that keep the terminals from touching in this position (like the SF123A's, see pic. 2) then you won't need any kind of insulator. If you packing AA's or any other cell type that will touch terminals when configured like this, cut a small square of inner tube (smaller than the diameter of the cell, but big enough to cover the majority of the contact, see pic. 3) and place it down in the tube on top of the first cells negative terminal so that they are insulated from each other. (pic. 4)
3. Align them so that there is about 3/4" (2cm) of tubing sticking out on either end. (pic 5.)
Step 3: Seal the Ends
2. With the end still rolled up, secure it with one of the zip ties. (pic. 2)
3. Trim the end of the tubing and the zip tie. (pic. 3)
4. Repeat 1-3 for the other end and your done (pic. 4)
Step 4: Deployment
Use a sharp knife to make a small slit in the rubber between the two cells (pic. 2) Try not to touch the contacts. Only a small cut is necessary. Bend the two batteries away from each other and the rubber will split apart allowing you to remove the cells.
Discard the rubber and you are good to go.