Really only ought to contain the following:
1. What "slow game"mistakes have you made recently?
Tactical (store the position where you missed the shot) or Strategic (write down what the book/stronger player suggested you should have played and why?)
2. Any general tips or guidelines that you found useful and are willing to apply to your own games.
The key to managing a "chess diary" is to constantly review it over and over again (once a week?) so that your brain slowly cements these patterns and transfers them from short-term to long-term memory.
If you can do this in a computer, try to make it randomly "quiz" you with these diary entries. If you prefer something non-digital, flashcards would work just as wel.
I've read that one tool for improving your game is keeping a Chess Diary or note book. What has been your experience? Have you found it useful? What type of information do you track in it? How do you use it to improve your game?