I love those bags of popcorn at the store but they're pretty expensive for what you get. Also, many times, the bags are huge and I feel obligated to eat the whole bag so that I don't waste it. Here's a solution:
Take one of those brown paper lunch bags. Put 1/4-1/3 cup of plain popcorn. You can get a big container of it much cheaper than the bags. Add any dry ingredients you want such as salt. Microwave just like you would the bagged popcorn. I always listen to my popcorn to tell when it's done. When it's done, add whatever you want. You can make all of those fancy popcorns you see in the store. Be creative.