This writing tip comes from David Sedaris' book Theft by Finding. Three quotes apply to the subject of keeping a diary:
When it comes to subject matter, all diarists are different. I was never one to write about my feelings, in part because they weren't that interesting (even to me) but mainly because they were so likely to change. Other people's feelings, though, that was a different story. Got a bone to pick with your stepmother or the manager of the place where you worked until yesterday? Please, let's talk!
If nothing else, a diary teaches you what you're interested in. (p. 3)
In order to record your life, you sort of need to live it. Not at your desk, but beyond it. Out in the world where it's so beautiful and complex and painful that sometimes you just need to sit down and write about it. (p. 9)
After coming home, I listened to the radio and cleaned up a little. A woman on All Things Considered wrote a book of advice called If You Want to Write and mentioned the importance of keeping a diary. It was valuable, she said, because after a while you'd stop being forced and pretentious and become honest and unafraid of your thoughts. (p. 94)