I've been going through the back posts of Silver Fox's blog, and boy, was I behind! She discussed the trusty yellow field book in this post. You know that I can't resist a gear post to comment on...
We have certain standards for our field books (which are generally either yellow or orange) based on legal requirements. They need to be bound. They need to be numbered, although we also number them ourselves as we go. Any errors must be crossed out with a single initialed line. They can't have skipped pages, or empty space at the end of each entry (you put a line through and sign the line) - all these so that you can't go back and sneak in more stuff after fieldwork is done.
So what do we put in those logbooks? Anything that may be important later. Date, day of the week (not everyone does this, but it really helps your memory when you're looking for something specific), weather, name and affiliation of everyone on site (and when they arrive and leave), name and time of any samples, names of visitors if they'll tell you (sometimes the activists are paranoid), deviations from the work plan, contact info (this either goes into the front or the back page), calculations, and anything else that may need to be remembered later. Project name, start and end dates, logbook number, and charge number go on the front, and the company contact information goes into the inside front cover (I tape a business card there to cover the "where to send if lost").
I've always ended up with a blizzard of paper logsheets (health and safety stuff, boring logs, well construction logs, sample logs), but the thing that ties the project together is the logbook. And well, it's rewarding to look back at your office (or the administrative file) and see a big line of bound books that you've filled yourself.