I jot down my recalled dreams as soon as I wake up. If I wait too long then they disappear on me as well.

I keep a book and pen or pencil right by my bed. When I wake up I jot down a few of the most prominent parts of any recalled dreams. I shower, eat, etc and then go back to that book later and write them out the dreams in full when I have the time.

You could try a recorder, wake up and dictate your dreams into it instead of writing them out.