Onions will keep for an incredibly long time if you tie them up in nylon stockings and hang them in a cool, dry place.
I have had onions last for months and months storing them this way.
I have not tried this with store bought onions but I believe that doing this would extend the life of those onions, too.
If you grow your own onions, you will first want to cure them in your usual way. Many people braid them. This year I did a lazy cure, just laying them out on some old door screens in a dry area.
When you are ready to put them into storage get some nylon stockings. I use knee high's because they are the right length for my storage space and my reach when I am retrieving an onion.
Push an onion down into the nylon stocking. Tie a knot above it. Then push another onion down to the knot. Keep doing this there is no more room in the stocking. Poke a hole in the top and hang your onion stocking on a nail in a cool, dry place.
To retrieve an onion for cooking just use scissors to cut the bottom onion off.
If you try this trick. I am certain that you will be surprised how long your onions last!