I log ALL my dives, even the ones where I don't actually dive. I did a nice long surface swim at Coral St., noticed the conditions were going from fairly nice to get-the-f-out-of-the-water, free dived for a pretty shell and got my butt out. Logged it, because even though it was fairly recent, looking back helps me remember things for the future like: "Diving may be really really important to you, but if you had the good judgment to get out of the water then, you can do it now".
I used to use the PADI pages in a Longs Drugs three ring binder. My dad (he's outfitting me with most of my gear; LUCKY me!) sent me the nice 30 dollar zippered dive flag log book, so I recopied my first dives, and now I'm using that. I really like that it has a lot of compartments to keep stuff, like pens, calculators, DAN mini-manuals, and so forth. Also, the pages that came with it included a "don't forget" checklist that included a LOT, which I thought was a nice feature.
What convinced me to be so anal about keeping my log books was going through my mom's log books from the late 70s. She logged well over 200 dives, so doesn't remember the details on those that were not in some way special. Reading her old books with little entries like "Water was WAY cold, but fell in love!" from when my mother was close to my age is pretty cool. I'd like to be able to pass my experiences to others (and myself!) once I've logged too many to remember the specifics.
Ishie