Even though it might seem like a no-brainer to you, enough people have commented on my little notebook that I thought it merited a short post.I rarely leave home without my little notebook:

• I have a medium-sized notebook so I can write little phrases on one line. But it still fits in my pocket.
• On the left-hand pages I write single vocabulary words. I suggest writing them with English on the left like this: "English word = yīngwén dāncí 英文单词". I haven't always done it like that but it makes it real hard to "look up" a word if my eyes are first hitting a bunch of pinyin. Also, if I'm walking, thinking my little thoughts, and I suddenly realize there's a word I want to know, I immediately write the English word and leave the Chinese side blank until I get the answer, like "escalator = " (it's the same as "elevator" by the way: "diàntī 电梯")
• On the right-hand pages I write phrases or little proverbs. Ideally (although I'm way behind), a native-speaker informant would record each of these phrases, as well as saying the page number, so I can memorize whole chunks of tonally correct Chinese.
• I laminated the outside cover of my notebook with packaging tape so that rain won't wreak immediate havoc on it.

Organize it however you want, but I strongly recommend carrying a field notebook whenever Chinese interaction, or even dead time that you might want to use as study time, is on the horizon.