How about focusing on a one time thing that's already happened in your waking life, before sleeping?

ie: I graduated from high school in october 2006 and I miss seeing all my friends together. I've only had two lucid dreams, the cause of them being that I was dreaming of being back at school again and seeing so many people I miss. So, school has become a dreamsign for me - whenever I dream of school, I know that I'm dreaming.