Iñigo Quilez (rgba) has some awesome rules for making 4k demos; the rules aren't that different from making a game, or other kinds of art for that matter:

1. Get some interesting effect(s)
2. Play randomly with all the parameters to get used to them
3. Think about something interesting to do with it (find an idea?)
4. Synchronize it to the mzk
5. People will think there is an underlying concept
6. Profit

I like the fifth point, it makes demo watching like cloud gazing; a kind of canvas to project your own imagination onto.