You might consider creating a file or note for every major project, campaign, or life goal you have. Then when you see something relevant to that, you copy/paste it in or make a note.
There's ideas I've done that would have been totally gone if I didn't have loose thoughts sketched into a file on them. It takes a bit of work to set up, but it's actually surprisingly easy and natural to update throughout the day.
Edit: I took a screenshot and uploaded it to Flickr - http://www.flickr.com/photos/sebastmarsh/5028168457/lightbox - that file was built out and iterated over five months, so definitely start smaller and let things expand. Feel free to email me if do something like this and have any questions on getting set up.