Was just discussing this yesterday with a colleague. I create new note for every idea I have. I explain it to any detail I feel at that moment.

I also add idea in a spread sheet, and then I rate them with the following criteria:

- Time needed

- Enjoyment

- Potential income

- Career impact

This makes it easy to see which one you are supposed to be working on. My current side project is the highest rated idea on the list.