how do you store popped popcorn so it doesn't get stale?

The best way to keep food as that is to put it in a container as airtight as possible. Exposure to the air will make it stale. Any storage should be DRY and do not freeze it as the freezing and defrosting will cause condensation that produces moisture.

Good candidates in order of preference:

-If you have those vacuum sealers/containers those are the best.. The vacuum selaers suck all the air out before heat sealing a plastic bag for excellent airtightness. The containers have a pump that lets you pump out the air to seal it in a vacuum.

-Tightest hard plastic storage containers with lids.
-Ziplock bags
-Your stomach
-Your dog’s stomach

Wouldn’t one of those vacuum sealers completely destroy something as fragile is popcorn?

I like to take a zip lock bag, put the pop corn in, hold it against my torso and zip it across to the last little bit and then run my hand down toward the popcorn as if I were smoothing out a wrinkle in my shirt. Then SNAP the bag shut. That works for smaller amounts.

For larger amounts I eat until I have a smaller amount.

I don’t. I sprinkle tuna oil through it and drive the cat nuts. He loves the smell but can’t stand biting popcorn.