I love this topic, so I'm really glad you asked this question. 🙂 I'll actually answer these out of order…

Where do I keep track of my ideas? Right now, I keep "big project ideas" in a spreadsheet, which allows me to arbitrarily filter or rank ideas based on any criteria I want to add. Thoughts that aren't really developed yet go in my "Getting Things Done"-style pile to process, to eventually get turned into big project ideas. Getting into the habit of writing everything down has really helped me develop…

Where do I find ideas for things to work on? …my habits for identifying these ideas and letting them evolve over time. Maybe it'll come out of a minor frustration, or a conversation with a friend, but I try to write down every "wouldn't it be cool if…" thought.

How do you decide what to work on? If I feel motivated to work on something, I try to pursue it as much as possible; conversely, if I'm not feeling like working on something, I'm not going to push it unless someone else is relying on the thing getting done. This makes me feel a lot more productive — I don't ever want to feel like, "ugh, I really need to work on this thing" — I just do stuff.