For me, I'm a college student and that means I have a notebook with me most places anyways, and if its not a physical notebook, its my TabletPC. I usually write notes down in a reserved section in all my class notebooks if I'm in class and something pops up all of a sudden. Same goes for my TabletPC which I have my own venture notebook that I will scribble thoughts down in. On the go, I usually just keep it in my head because I don't yet have my own smartphone. Later in the evening after I've taken these notes I usually sit down and flip through my new ones and look at what I was thinking and figure out if its something worth expanding on or not. This goes a lot back to how you manage your time and the "shinny thing syndrom" (ADD).

It's a system that works pretty well for me, but everyone's different. I'm the kind of person who, even while other things are going on around me, am looking not only at the current state of my current venture but the direction and potential of that venture and others. Needless to say, I get a lot of those "oh crap, that's neat" when I'm out and about. The important part is really how you make use of those notes later on. I go through them on a nightly or bi-nightly basis and just browse through some of my ideas or concepts and figure out what's worth keeping track of or expanding and what's something to set aside as "good idea, but not now." It's important not to overwhelm yourself with notes going in a different direction.

The notes that are worth keeping and expanding I usually stick all together in a folder of some sort and they usually end up typed, expanded, and analyzed in a word document at some point in time where I've fully expanded on or refined an idea or perspective.