Have you ever seen one of these?
It's not really a sewing tool. It's for cleaning out a musical instrument, namely a flute. Like so:
Naturally, I had this thing already hanging around, because I use it to clean my flute. Indispensable.
As a sewing tool, though, it's a luxury I no longer can live without. I abhor using a safety pin to thread something through a casing. Ugh! Takes forever. This takes a few seconds.
Also handy for threading a camera strap through a cover. :)
Your local music store will stock a metal cleaning rod for a few bucks, or a wooden one like mine for a few more. Metal rods are skinnier. Find one online here.
Trust me on this one. You'll wonder how you lived without it, especially if you make a lot of elastic waistbands or aprons with really long casings!!