Journaling is not a "biblical" discipline. Neither Jesus nor the apostles did it the way we practice it today. But it does compliment what we are commanded to do, and is so personal and customizable it can work for almost anyone. For the Christian, a journal is a place to write down God's work in your life. It may be filled with accounts of your days, or perhaps your thoughts on a passage of Scripture. It can be a collection of your prayers, goals and accomplishments. Here you can write down your thoughts about God, Jesus the Christian life, or what God wants you to do based on what he says in the Bible. It can all of these things, just one of them or something else.
Don Whitney argues that journaling is profitable because it can help us in several ways: It can help us evaluate where we are at spiritually, meditate on Scripture, express/articulate our thoughts about God, remember God's work in our lives, stay connected to the other spiritual disciplines and more ( check out chapter 11, Spiritual Disciplines for the Christian Life). Journaling helps us articulate what God is teaching us, which then allows us to more comfortably communicate that to others. To share what God is doing in our lives, is one of the best forms of teaching/encouraging.
Maybe you're struggling spiritually at the moment. Maybe things seem dry, or cold, or hard. That a great time to pick up journaling. Start by writing down exactly where you are, what you are feeling, and where you want to be. Write out your prayer for revival, and begin chronicling your pursuit of God. Like many of the Psalms, half of my journals start that way, but they end in a much better place.
Many of you already know about Moleskine (mol-a-skeen'-a) journals and sketchbooks, but I only discovered them this summer and wanted to pass on the find to those who, like me, have been in the dark about them. I have kept journals off and on since 1993. I have a stack of spiral notebooks and notepads on a shelf next to my desk. There are a few leather bound journals I received as gifts, but I have always preferred the cheap ones. I am now using a Moleskine and have found it to be perfect: the size, binding, pocket in the back, acid-free paper (and it's romantic history doesn't hurt either). You can now buy them at Barnes and Noble, or order online through moleskineus.com. If you don't want to spend $15 on one of these, just get a spiral. Your spiritual life wont know the difference.