First off, your question made me chuckle, as I did not know what "dairy" writing was... do you write on a cow's back or something? Amazing how a typo can change the meaning of the question eh?
Anyhow, to answer your intended question, I do not keep a diary. I do, however like to journal my thoughts. It is not a strict daily exercise, but rather more an as-and-when set up. If I encounter something that influences my life I write it in my journal.
Numerous times I have gone back over old journal entries and re-learned the point that I truly thought I had already learned. It goes to show that great truths and revelations are timeless.