Well, if it's anything like those digital watches and other such things that use the same little silver batteries, you will probably be able to tell when the batteries get weak because of faded numbers and such. So if you see that, replace it immediately.
Also, there's a myth that my friends use that if the batteries die in something, and you really need to get it to turn on one last time,(security tokens being a perfect example of this) you can try putting it in the freezer for a bit, and it might be able to rouse that one last bit of juice out of it. Not sure if that myth is true, but I've seen it work, and I know that colder things conduct electricity better, so I guess it's not based on wild theories at least. Still wouldn't rely on it, but like I said I have seen it work before with cheap phones and stuff like that.
Oh and never store the token in the sun or in a very hot place. Batteries normally self discharge at a couple percent per year at room temperature, but if stored at 90-100 degrees or higher, they can discharge 25%+ per year, and the sun adds like 20-30 degrees. Doesn't matter for me, since right now it's the middle of winter, but if you live in a warmer climate or maybe during the summer, it's something to keep in mind.
Lastly, the things are pretty cheap... If you have been using it for years, maybe just replace it occasionally. I heard from people in my linkshell that they've seen them die anywhere from 6 months in to 3 years. You can probably make them last a lot longer if you press the button a second time to shut it off after you're done, instead of letting it time out and turning itself off.
I have no idea why I know that much about batteries, but whatever, hope it helps ya'. :P