Never go to zero
Obviously, using your battery is going to make it degrade. But it's going to slowly die even if you just leave that iPad in the closet for a bit. There's a trick to minimizing that inevitable aging though: leave it a little bit of juice.
If you're going to be shelving any lithium-ion battery for a long time, try to leave it with at least 40 percent battery power to tide it over. Lithium-ion batteries don't hemmorage power at 30 percent a month like nickel-metal-hydride batteries do; they'll lose maybe five to ten percent of their charge each month.
And when lithium-ion batteries get too low—like, literally zero percent—they get seriously unstable, and dangerous to charge. To prevent explosion-type disasters if you do try to charge one, lithium-ion batteries have built-in self-destruct circuits that will disable (read: destroy) the battery for good, if it reaches rock bottom. And sure, that'll save you from a face full of battery-acid, but it'll also leave you short one battery.