Honestly there's no rhyme or reason or system to it. It's just a fat 3x5 notebook that I write in with a pencil. To keep track of the ideas means I have to flip back through it and read what I've written. Occasionally, I'll cull through it and make new summary lists to consolidate and remove things I've already done.
Here are three things that have "evolved" in it over time that I really like:
A small list of "current" themes or styles or kinds of subjects. I keep these in the back of my mind when I'm out or driving around. Now APUG MSA topic is added to these!
Pullouts near my house with great views, and what direction they view, so if the sky is awesome in a particular direction, I can quickly choose a good close-by place to make a photo.
A "to get" list, which is where I write down potential photos I see or think of. This list has gotten long, so I never have any lack of ideas. Some of these are exciting but would require just the perfect light...
I don't want to get to rigid or mechanical about my hobby, and the "to get" list is more like a thing to fall back on if I'm uninspired. Mostly I just carry a camera wherever I've chosen to walk and don't worry about it if I don't end up using it.
Ah right! I use the ideas book for everything , not just photography!
My to get list is all messed up, should get it consolidated!