Some pieces finish themselves, or cry out to be finished, and that is motivation enough for me. Other pieces (usually the ones I leave for a while) require me to find a place for them, such as a hub, or a story anthology. Then I use that as motivation to finish the piece. This is particularly true of pieces that I am polishing.
That said, I have dozens and dozens of fragments and unfinished pieces on scraps of paper on my desk as well as in files in my computer. Many of those pieces will not be finished or will be pulled into a bigger project that I will work on later.