I keep a notebook on my nightstand. If I happen to wake up after a dream, I reach over and fumble for my pen and, usually half-awake, write down keywords and short descriptions of my dream. I include colors, scenery, dialogue, characters, strange objects, anything I can recall.
First thing in the morning, I create rough sketches based on my notes, and then develop the finished piece using pen and ink, sometimes watercolor. The result is an accurate depiction (accurate as I can be) of my dreams from the night before.