How do I "Lucid Dream"

Funny enough I experienced one last night. I was just lying there thinking of random junk and started picturing things in my mind. I basically forced myself to have an awake dream but then I fell asleep while I was still conscious and was able to control my dream.

It was basically a thought that slowly developed into a dream which got more and more vivid until it became a full on dream.

Hope this made sense.