When I'm feeling that sort of obsessed, I take notes: LOTS and LOTS of notes. In part because I don't get a lot of time to actually sit down and write coherent prose, but I don't want to forget anything between the time I've had the idea and the time I actually get a chance to incorporate it into the story. But part of it is also because I come up with lots of stuff while I'm writing that doesn't end up making it into the story (as you point out too), but I still want it to exist in some form for me, at least. For the most recent longer novella I wrote, I have a file of about 8500 words of notes that never made it in (!).

-J