This is one of those things where you feel stupid after you hear it, because it's so obvious you can't believe you never figured it out on your own (apparently my fiance has been doing this for her kids for years).
1/4 cup popping corn (I paid $1.50 for a 32 ounce bag at Wal-Mart)
One of those small brown lunch bags (you can get packs of 50 for like $1)
Dump the popcorn in the bag, fold the top over a couple of times. Stick it in the microwave and pop, on high, time varies, just listen until 1-2 seconds between pops (mine took about 2:45). Some people say staple the bag, but it's unnecessary and there is the possibility of fire. Some say use 1/2 to a full teaspoon of olive oil - I found it made no difference other than a greasy bag. Add the oil/butter and salt/seasoning after and just shake the bag. I did find notching out a small hole near the top helped let the steam escape and made the popcorn slightly crispier.