At the moment, I have a 20 page file of "Things I'd like to do someday" as well as a notebook of "Cool snippets and ideas."
Whever I come up with a small idea on the road, it goes into the notebook. Whenever I come up with a more complex idea that needs explanation, or clarification, I break open the document and begin writing everything that comes to mind as quickly as I can until nothing else flows. I edit it for half an hour to an hour afterwards, then call it a wrap unless I get re-inspired by that idea again later.
If I need a villian, or an interesting situation, I dig through the notebook. If I need the cornerstone to a plot, or a new campaign, I dig through the file.
Sometimes, if I'm short of things to do, I'll dig through one or the other until something catches my eye, then run with it.
I find that if I don't write it all down, I tend not to remember them later.