For awhile, I gave in to the insistent demand of the newest, shiniest idea...and then never finished anything. In order to finish a novel-length work, I have to not only be disciplined in my life (kid comes first, husband a distant second ;), housework, basic sanitation and socializing with friends who complain they've forgotten what I look like all jumble together), but also in my head.
Now, I pull up a fresh word doc and dump the idea, the scene/dialogue, or a couple pages of outline into a file, slap a name on it and save it in a folder called "Ideas and Starts". Sometimes, those scenes will find a way into the ms I'm working on. Otherwise, they have to wait until I finish my draft or stage, at which time I'll sift through and start on whichever one is calling the loudest. The strangest part is when I open one of them and have no memory of having written it, but gosh it sounds like a great/clever/cool idea. That's a rare delight.