Ubiquitous capture is a great term from Getting Things Done. Like the best ideas from GTD, it is simple, obvious in retrospect, but changes everything. Ubiquitous capture means: When you think of something, you should write it down, right away, in some place where you will check it later.
This is especially good for keeping track of ideas for new research projects. I tend to find ideas for new projects while I'm walking to work, when I'm sitting in a talk, or when I'm working intensely for a paper deadline. Hardly ever can I work on them right away, but I know that I will need them later. So, whenever I have an idea for a new project, I stop whatever I'm doing and write in down in my ideas list. If I have to stop in the street or pause a one-on-one meeting to pull out my phone, well, a benefit of being an academic is that you get to be eccentric.
I keep my ideas list in Evernote, but it doesn't matter what you use, as long as all your ideas are on one list.
Later, usually many months later, a student will ask me for suggestions for an undergraduate, master's, or PhD project. I go back to my ideas list and look. I also tag each idea "ug", "msc", or "phd", if I think it would work well for one of those degrees.
I also look back through the list periodically to pull out ones that are especially exciting. Every idea is exciting when you first have it; the ones that are still exciting a week later are the ones to keep.
Of course I use a similar system for blog ideas.