Find a long piece of music or an album, preferably instrumentalist. Ideally, it has highs and lows - parts where the music is upbeat, parts where it's dark, sections of tension, melancholia, light relief. Classical music works well as do movie soundtracks (if you skip out any pop songs), or (my personal favourites) albums by Vangelis, Jean-Michel Jarre or Enigma.

Sit down, put the album on, and start writing whatever scene the music makes you think of. A tense, underworld exploration? A happy couple? A chaotic storm of dreams? Go with the music, shifting the tone and pace of the story to match as you listen.

If you're someone who types slowly, you may find you have only time for a sentence or two before the tone changes - that's okay. Treat it as an outline exercise, in that case - you can flesh out the sentence into a paragraph or so later.