For really long life, never below 80%
Not practical, sadly, but if you've ever wondered how the batteries in a satellite last through literally tens of thousands of cycles (see LEO satellite eclipse experience), or for that matter how the Mars Rovers keep ticking after 5 years, it's because their batteries are sized such that they're never substantially depleted. They're never run down below where they have perhaps 80% of nominal capacity remaining. Less movement of "stuff" in the battery equals longer preservation of cell geometry.
Unfortunately that would mean a huge battery pack for a laptop if you wanted to use it for more than an hour or so.
Hubble engineers discovered a few years ago that conditioning the batteries on the telescope was actually ruining the batteries. Since abandoning the conditioning process the batteries on the spacecraft have lost little if any capacity, after losing something like 30% largely due to following conventional wisdom on conditioning.
A wonderful battery technology we're leaving untapped is simply improving the quality of software construction so that we don't need to have bomb-like batteries to accomplish things that would need less juice if software were better made...