It is the wandering mind which seems to come up with ideas for stories. I, and many other writers and illustrators, seem to get most of my ideas whilst on public transport. It must be something about having unstructured time, when thoughts are free to roam.
I always have a sketch book with me to jot down ideas. So my Top Tip is a bit of a strange one: don’t play with your mobile phone on the train. Imagine if J.K. Rowling or Hilary Mantel had been playing Candy Crush on their famously fêted railway journeys!