NeAvO's right.

I keep an open small memo pad with a pen clipped to the spiral binding next to my pillow. When I wake up from dream, I don't turn on the lights, nor open my eyes.. I just reach for the pad and start writing.

Also, you don't have to write a "book" about the dreams you had in your journal. For me, if I'm too tired (or lazy), simply writing down the main topic/subject, and a word or two or a feeling is enough for me to recall a dream.

Some people just write snippets of their dreams.. others write paragraphs, some put images, or feelings, others draw pictures.. whatever helps to bring a dream back to you when you read over it the next morning is appropriate.