I save all off the zipper or snap bags that stuff like sheets, blankets and comforters come in. Lot's of stuff in the baby section comes in these bags...lots of shapes and sizes.
I recently organized all of my UFOs. I wanted to know how many I had. I put each one of them in one of these bags with the fabric, blocks, pattern etc. It helped seeing them all, so I could start to think through how to finish them.
All of these bags are kept together (stacked neatly) and I grab a bag when I am ready to work on one...it makes the whole finishing process less daunting.
Of course large zip lock backs would work but I just recycled what I had.