Be aware that the battery level shown is an APPROXIMATION only. There's no real way to gauge how much energy is still left in a battery (it's not like the fuel tank of a car; there's no "liquid" to measure.)
Solution: just use the mouse until it goes dead, and never bother with the battery indicator (I've had it show a negative number for remaining power......) Then just change the batteries. To make it last a bit longer you can use the power switch at the back, but in my experience that's just a waste of effort really; my magic mouse still lasts for many weeks on one charge even with the switch left in the "on" postion the whole time. I only switch the mouse off when I'm transporting it, so it doesn't wake up my computer while still packed in the laptop bag.
Others have suggested rechargeable Eneloop batteries. I use these too; avoid using very high rating NiMH batteries in stores; these typically have very high self-discharge rates and will go flat inside of a few months even if you don't use them at all. Eneloops will hold their charge for a long time, and is thus better suited for use in low-power devices like mice and keyboards and such where you don't want to have to change batteries all the damn time.
Apple's own battery charger comes with six Eneloop cells (re-badged); it's not the best charger in the world maybe (or the cheapest, lol), but it's decent enough, very small and easy to pack in a bag, and it works. I use it myself to re-charge my magic mouse's batteries.
