Really! Same here! Most of the time almost had my ideas pop in my brain suddenly wipe out until SOME people made an rude interruption my work.
After all, I still remember what is my ideas were in my stories in RWBY x Naruto X-Over story.
First was, trace back your mind to recalled of anything you had previously seen.
If happens, make a note on Docs, includes Microsoft words in app and Facebook (includes a image on depends like a USB or so), to list out your ideas encase that happens.