The short answer to this question is: buggered if I know, but I’d like to suggest a few things that might help unearth a nugget or two.

1. Open your eyes and your ears. Expose yourself to as much stuff as possible.  Whether it’s reading magazines, or novels, or listening to podcasts, or eavesdropping other people’s conversations on the way to work, the more you read and hear, the greater your chances of coming across something that captures your imagination.  I’m not suggesting you copy other people’s ideas, of course, but it’s amazing how our brains can process things and lead us off in unexpected directions.
2. Take a notebook with you everywhere. Write stuff down, or you’ll forget it.  Yes, you will.  If not a notebook, have some other way to capture your thoughts.
3. Be lucky. Sometimes serendipity plays a part.  One of the core ideas behind Paradise – really the one from which everything else sprung – occurred to me while I was sitting in church at the funeral of my wife’s great-aunt Ethel.  Half way through the service, four brothers stood up and sang “Abide With Me” in pitch-perfect four-part harmony.  And while I should have been thinking, “Poor old Ethel, she had a good life, I’ll miss her,” what I was actually thinking was: Oh my God!  Singing brothers!  That would be a perfect idea for a book!  So, sorry Ethel.  But thanks.
4. Learn to spot the good ones. Maybe the hardest trick of all is to differentiate between the good ideas and the merely so-so (and the downright dreadful.)  Here’s where instinct is invaluable.  It goes without saying that if you can spot the clunkers early you’ll be saving yourself an awful lot of heartache later on.  Don’t be afraid to ask other people what they think.  Better to know now than 30,000 words in.  I speak from bitter experience.  I wrote half a novel about the Algerian War of Independence which was set in Paris in the 1960s before I realized that I was probably the only person on the planet who found the topic interesting.  It was Bruce, my agent, who eventually put me right.  He read the first three chapters and sent back the following email: “Dear Alex, I’m sorry, but this won’t do at all.”
5. Be original. There are plenty of good ideas out there.  Some of them have your name on them.   Above all, resist the urge to copy (or worse, steal) someone else’s idea.  Quite apart from anything else, if it’s not yours you won’t believe in it, and that will show through in your writing.
6. Be patient. In two ways.  (a) You’ll have to wait for the ideas to come.  Don’t fret if you are uninspired.  Just keep digging (see 1 above) and something will come.  (b)  Don’t assume that every idea has to be used at once.  It may be literally years before you work out how an idea could fit into your work.  Keep a careful note and (if you can) some kind of retrieval system so that you can review old notes and ideas.  Some of the bits of business that I have used in Paradise have been knocking around my head for ages… only with this book did I work out how to use them.  Don’t try and cram too much into whatever you happen to be doing at the time.  If it doesn’t fit, don’t force it.  Allow ideas to develop before you commit them to paper.
7. Be realistic. Let’s be honest about how this works: by and large, unless (apparently) you’re J.K. Rowling or Stephanie Meyer, the plots of novels will most likely not plop, fully formed, into your head or occur to you in dreams.  Crafting stories is a long process.  Be a magpie.  Hoard ideas, snatch scraps from wherever, and be ready to discard and reconstruct as necessary.  Compose, in the original sense of the word: put things together.

Right, that’s quite enough bossiness for one day.  Here endeth the lesson.