For flashlights which I rarely use but are important to have ready, like the ones in my cars, I change the batteries twice a year, when we change the clocks for Daylight Savings Time. Those lights are loaded with alkaline AA's. The interior of a car is not a forgiving place to store anything, whether batteries or water or even a ballpoint pen, so I would rather be conservative about having "fresh" batteries in the light. I try to check that the light works by turning it on monthly, but I don't actually unscrew it to see if the battery is leaking. Leaking batteries is something most of us have probably experienced at some point, and they tend to ruin your lights, so that's another reason that I replace the batteries twice a year even if the batteries were hardly used.

I could use lithium AA's in those car lights and probably just let them sit longer without any checking, but I feel more confidence in the batteries if I change them more often, so I just use cheaper alkalines and change them twice a year. The old batteries then go into a radio and will eventually be used up, so it's not wasted.

For lights which I use more regularly, I use rechargeable batteries. The Sanyo Eneloops which bentirran mentioned are excellent for flashlights which take AA/AAA batteries. I hear that they will soon come out with C/D sizes, which will work for powering those common C/D-size Maglites for a good long time. NiMH rechargeables are becoming very useful alternatives to alkalines or lithiums, now that low self-discharge chemistries are becoming available.