I keep notes, sometimes multiple notes, for each work / fic I have brewing. Whenever I have a good idea, be it a line of dialog, a scene, a personality trait, plot twist, *anything*, I immediately go to the computer (or nearest piece of paper if the computer is out of reach) and write it down. Immediately. That's the important part. Do not pass Go, do not collect 200$, go straight to the nearest writable surface and make notes. Make it a habit.
Later I'll organize them into things like backstory / world building, plot points, and scenes, and toss out or rework conflicting ideas.