Here are a few books on the subject of Journaling.... (in case you want to read instead of write).
The Many Faces of Journaling : Topics & Techniques for Personal Journal Writing
by Linda C. Senn
This book starts out with some historical perspective on Journaling and then looks at different types of Journaling with all the reasons for each.
Notes from Myself: A Guide to Creative Journal Writing
by Anne Hazard Aldrich
This book is more about the mechanics of Journaling... what you need to do... and then has some background later.