For a few weeks now, I've been using a Pocket Filofax as a diet and fitness journal. For the curious, here's how I use it:
1) Week-per-2-page diary. Each day, I log exercise done (with heart rate), cups of water drunk, weight/body fat (on the days I weigh), and so on. The illustration here is a template page I made, to keep things clear by writing the same information in the same place on the page each time. The pages are cramped, so I did some planning before I started writing in ink all over them.
2) Eating journal. On yellow paper, one side of one leaf per day. I write down the day's plan the night before, and then...er...edit the next day based on what I actually ate.
3) Notes pages. Blue and pink paper for various kinds of notes and quotes from the program I'm using, The Beck Diet Solution.