How to make up stuff for a living

I do often get asked where I get my ideas. (And sometimes the tone implies a certain concern for the inner workings of my brain.)  I love Neil Gaiman's take on the question. He's tried the flippant answers: Idea of the Month club. A corner shop. Pete Atkins.  Now, he just tells people the truth:

'I make them up,' I tell them. 'Out of my head.'

What most people want to know, though, is how to do that.  Every writer has a different way of coming up with ideas. Actually, the idea isn't the hard ...