Adapted from Alton Brown's Plain Brown Popper recipe, we've started making our own popcorn bags for the microwave.
DIY Microwave Popcorn
Brown paper lunch bag
Good quality popcorn
A little more than 1 tsp. safflower oil (or oil of your choice)
Toss the popcorn in the paper bag lunch bag, and sprinkle with a little more than 1 tsp. oil. Shake bag lightly to distribute the oil. Fold the top of the bag over twice and tear a slit in the top of the bag. Fold each flap of the bag over in opposite directions. This will keep the bag closed during popping.
Place the bag in the microwave on popcorn setting, adding an extra 30 seconds onto the end of the popping time. Listen for the popping to slow down to turn off microwave.
If you don't have a popcorn setting, microwave on high for 2 minutes to 3 minutes, until popping slows. Press stop when there is about 1-2 seconds between pops, any longer and it will burn.
Use popcorn salt, and/or drizzle melted butter over popped popcorn. Do not add butter to before popping, it will burn.
Note: Edited to add image