I'm not sure exactly where I picked up this trick, but it certainly saves on popcorn expenses. Even when purchased on sale, the cost per bag of microwave popcorn is pretty high. A standard bag is too much for me, but the leftovers go stale by the next day. The little single serving bags are even more expensive.
This method is about as cheap as you can get - a bag of popcorn kernels, a bag of plain old brown paper lunch sacks, a little oil and a microwave are all you really need.
Open a paper bag fully. Add about three tablespoons of kernels; if you add more than that, most will not pop before the rest begins to burn. Pour a little bit of oil over the kernels in the bag. If you have one of those wonderful little oil spritzers (essentially a spray bottle you fill with vegetable oil), they work like a charm. You want enough oil to moisten the popcorn a bit, but not soak through the bag. Less is more.
Fold the top of the bag over twice; shake a bit to distribute oil. Lay the bag on its side in the microwave. Zap at high power for about four to five minutes. DO NOT WALK AWAY! It takes a minute or so for the corn to begin to pop, but once it goes, it goes quickly. It also burns very quickly; be very careful how long you leave it in. You cannot wait for the popping to stop (or, more likely, the smoke to pour from your microwave) as a signal to take the bag out of the microwave.
Dump the contents into a bowl, avoiding grabbing the bag at any grease spots, and douse with your topping of choice. There will be some unpopped kernels, but no more than with conventional microwave popcorn.