Don't you walk into a room and just notice random things, which could have interesting uses, or trigger a whole story in your head, or an unusual association?
Or imagine all the possible ways, an important converstation could go. The multiple response you could get during a speech.
A good idea is plan a holiday, and imagine all the possible scenaros you coud be in (it's a bitch packing when you think like that though). It's like breathing to me, I just do it.