I keep a dream journal right near my bed. I date it for the morning I wake up on.
This is my general dream journal layout:

TITLE DATE

VIVIDNESS: _/10 LENGTH: __ days/hrs/min/sec it felt like in the dream
EMOTIONS: (ex: happy, sad, jealous, depressed)
SETTING: (ex: my house, the park, my school, france)

then I write down my dream.