Do you get cold toes in the winter ? Does your girlfriend want to stop skiing before you do because her feet are freezing ?
If so, you may have considered battery-operated electric shoe heaters, but been put off by the price of those made by companies like Hotronic and Therm-ic, whose systems cost up to $200, and in 2005 Therm-ic still use NiCd technology, with its poor cold-weather behaviour, sensitivity to sloppy charging habits, and the disapproval of the greenies.
(PS 2023 : 15 years on, technology has advanced, and I now use products from Bucken, which are great, but still expensive.)
And then maybe you were put off the idea of building your own because of the difficulty of making the heating elements reliable and comfortable. Well here's the compromise I made - I bought a pair of insoles with embedded heating elements from Therm-ic, which are very well made and comfortable, and only cost $40 in an expensive Swedish ski resort. Then I built a simple battery pack to drive them for about $10, saving $150 on what they wanted to charge for theirs, and getting superior NiMH technology to boot. The insoles can be easily moved around between shoes.

Materials :
To make the battery pack, all you need is :
a battery holder,
a switch,
a plug and socket (for example 3.5mm audio jacks),
bits of wire.

I chose a holder for two AA cells, and that seems to be enough for my girlfriend's feet on a day's outing skiing or skating. You may prefer a 4-cell holder at the expense of the extra volume and weight.

Construction :
Kind of obvious if you've got this far, but connect the socket to the battery holder, taking one of the wires through the switch on its way. You can then glue the switch and socket on to the battery holder so that you have something that looks like the first picture. Note that the holder is probably made of a 'fat' plastic like polythene, which most glues won't stick to. Use something like Plastic Padding's SuperAttak. Then change the plug that comes on the end of the insole cable to one that is compatible with your socket. In the second picture here you see a conversion cable, because this was a prototype before I had the courage to cut off the original cable and make it neat and tidy. If you are lucky, you might even find a ribbon cable connector that is compatible with the original plug, and you won't need to cut or use an adaptor. If you do this, you may want to cut a little groove in the original plug so that it fits more snugly to the ribbon connector.
Now you can put this assembly into a little fabric bag that you can strap on to the back of the shoe or boot.
Simple, huh ?
If you want to make things a little more complicated, you can use 4 batteries and have a switch with which you can choose to have all 4 in series (very warm, short battery life), or two pairs in parallel (not so warm, longer life). For this, you need a double-pole double-throw (DPDT) switch, and wire it as shown in the figure.

The resistance of the Therm-ic insole element is around 10 ohms, so with 2 AA NiMH batteries in series, the current drawn is approximately 200mA, and the energy output around 0.5W, giving a lifetime of around 10 hours between charges. With 4 batteries in series you get 400mA and 2W, with a lifetime of 5 hours.

ToestieFingers
As a start, I made a proof-of-concept prototype using half a meter of 20 ohms/m Kanthal® wire and sewing this into the fingers of some thin liner gloves, and driving with the same 3v power source as in the toesties. This didn't work well, perhaps because the heat was distributed over too large an area, so I tried a 9v battery instead, and this was very warm for 15 minutes or so, but then I realised it was drawing 1 ampere, and so drained the battery too fast. Also, the uninsulated wire shorted out in various places, resulting in badly distributed heat, and being single-core it was hard to sew into the glove and had sharp ends that inevitably poked into the skin. Plus it doesn't take solder well.

So I was delighted to discover Cooner Wire who sell every type of wire imagineable, and many that are not. Product AS636 is a 32-gauge multicore stainless steel wire in a PVC sheath that is only 0.2mm in diameter, and flexible enough to sew with a normal needle.
The resistance is around 30 ohms/m. Although in principle hard to solder, if you wrap some multicore copper wire around the joint and add a generous amount of solder, you get a very good joint with a normal electronics iron. I don't know how much the wire costs, but probably not much because they were kind enough to send me a metre as a free sample when they heard what I was doing. Unfortunately this was a bit short for a pair of gloves, as 50cm of this gauge has a resistance of 15 ohms, and two 25cm lengths in parallel less than 4 ohms, and my target was around 6 or 7 ohms. I went with the 4 ohm solution, and that was very warm, but two AAA batteries only lasted 30 mins, two AAs 90 mins. If I'd had another metre of wire I could have gotten 3 hours with the AAs, and still reasonably warm.

Still, it seems that there is a fundamental problem with gloves that it's uncomfortable to carry as many batteries as you can tolerate on your feet, and a pair of AAAs or even AAs just doesn't hold enough energy for a whole day. One solution to that is to have cables running up your arm to bigger battery packs, but that's too annoying for my client. An elastic pocket will be added to the outside of the glove's wrist to hold the batteries, and the idea is to use this glove as a liner inside a bigger glove.

Battery info
I am using GP rechargeable NiMH cells with the following specs :
AAA 850mAh 1.2v =~ 1 Wh
AA 2500mAh 1.2v =~ 3 Wh
PP3 170mAh 8.4v =~ 1.5 Wh

Toesties^MT is an unregistered marktrade.

© Mark Harris 2006, but feel free to distribute without changes.

Bonus paragraph :
Soon after publishing this page I got an email from the ATF bomb squad in the US, asking me to call them. Very curious, and a little apprehensive, I called the number expecting some military type to bark at me that I should stop encouraging people to put wires in their shoes. Instead I heard "Hello ! This is Lilly". It turns out she just wanted permission to publishes this page in an advisory to make officers aware of some of the things out there that are not bombs. So rest assured that you won't be taken away to a remote area and blown up in a controlled explosion just because you are wearing toesties. Not if everyone reads their advisories at least...