The more affirmations the better, however its alot more easier to just keep a dream journal and write down the important dreams. Sometimes they might re-occur and you might find a chain link between those dreams.
In order for your dreams to enter the long term memory, you need to redream it consciously and place your own emotions to it. Feel what happened inside that dream, and how it impacts your health or state of being. This way it will be alot easier for you to recall the dream after 2-3 years ;)