Apple Users

Sanyo's Eneloop batteries would be a good choice, as you would be able to charge them as many as 1500 times.

You might try buying a set of four batteries along with a charger made for Eneloops.
With your mouse, you can check your battery level by clicking the bluetooth icon in the upper right corner of your screen. In the drop down menu you will be able to select Apple Magic Mouse and its battery level.

Once the level is low enough, I'll turn the mouse off, take out the old batteries and insert an already-recharged 2nd set. I'll then immediately charge the spent batteries and tuck them away in a drawer to be used again a few weeks later. I never turn my mouse off except to change out batteries.

The Ni-MH type batteries retain 75% of their charge capacity after as much as 3 years storage.

Please forgive me if you were already familiar with Ni-MH rechargeable batteries.