I have a method of storing all of my "good" map ideas in one place to browse through later. And I thought the community might appreciate the idea and be able to use it themselves. So what I do is when an idea sparks the imagination but doesn't currently fit into my project I will open my map 'notebook' and place a few key pieces to remind myself later about it. This isn't to tell me how to build an entire map mind you, but its really useful for small sections or mechanics that I think would implement well under the right conditions. So in the future, use a notebook. Sometimes this is better than writing it down somewhere.
I make entire maps around a small concept, not one map with a lot of concepts, but basically the same thing.
I just draw the blueprints for my maps in a graphing notebook I never used in high school. But I guess this could work too..
I have a folder. It contains concept of maps, and tricks I would use; and it also contains birds-eye views of maps I want to make.