Writing anything down makes it stay longer in your mental disk drive, which is why teachers expect students to take notes, and gurus encourage people to keep journals and to write affirmations.
Think of the mental power it takes to write something. First, a large part of your brain is used to control the many muscles of your hand, and writing is a fine motor skill that takes great control.
Then, you have the part of your brain that manages your language center. Almost everyone says the affirmation as it is being written. Even if there is no audible vocalization, the speaking muscles contract as if the person were speaking, and we hear the words in our minds, again activating many brain functions.
As we write, we see the results of the writing, activating the visual cortex. So writing an affirmation involves sight, speaking, hearing, left-brain language and right-brain awareness of use of space on the paper as we write. The message is stored in five different areas, connecting many synaptic pathways.
Writing down affirmations uses five times as many brain functions as merely reading them or even saying them aloud. Memorizing the affirmation, and actually vocalizing it as you write makes a stronger impact.
Of course, the reason for repeating any affirmation is so that you develop a belief in it, since a belief is only a thought that you have had many times. Why not put the power of your whole brain behind the learning of a new belief by not only writing your affirmations, but writing about them, talking about them, and listening to yourself say them?
What have you to lose but those old, thread-bare beliefs that have worn down to the inner ‘lie’?