I probably don't have to mention this since I know that Danny Gregory already had quite a following, but I recently finished his book An Illustrated Life and I have to say I'd recommend it to anyone who's thinking of starting (or already has) an art journal. Definitely illuminating to see the various ways that people journal. Some are so precise, and others are very free. If anything, you get a real message of just do it. Stop worrying, stop fussing, and just DO it. Maybe if I hit myself over the head repeatedly with the book I'll remember that.
Also, I was wondering if anyone out there has done or is doing Wreck This Journal? I just started mine a few days ago, and I have to say that it's altogether too much fun. Even if it does go against every single instinct of mine to buy a book specifically to destroy it... but then that's entirely the point, right?