How to keep up with the flow of ideas, without abandoning current project is the Fantastic Friday Writers topic.
I have yet to master this. With a strict time table, I have been able to work on multiple projects for short spurts of time. But as a novel begins to consume my creative energies [which they always end up doing], whatever else I was doing usually gets chucked to the side. I have a pile of partially written projects waiting on my return. When I realize I won't be back for awhile, I go back in and quickly jot down my remaining thoughts on the project to keep my place and jog my memory.
I keep an idea journal as well. Anything I think could be the kernel of a story idea, or become part of a story, gets written in there.
I have an idea of typing out rough sketches and outlines, but haven't done that yet. I do that when revising my novels - write out brief outlines and sketches. I have yet to do it with new story ideas, but will schedule in some time each week or every other week to do that. Now that I thought of it.
PS, I had high hopes of fixing my commenting troubles yesterday only to have them dashed. I see Blogger has had time to create new doohickeys of which we have no idea what they do, but can't fix their problems -- like folks being unable to comment on some blogs or slow loading of the widgets. I've about given up all hope on ever being able to comment normally again.