before you fall asleep tell yourself that you are going to remember your dreams.
make it a strong statement to yourself and be prepared to wake up after you dream. be calm and relax and let your conscious mind accept the images and commit them to memory.
have a notebook handy and jot down the experience when you awake.
As far as remembering something from say last night, im not sure, I would relax, become very calm and try to reach back. then jot down your impressions on paper.