You can add AppleCare to the notebook any time during its initial year of hardware warranty coverage. Wait until you're getting close to that year before doing anything. If you're still using the ibook and don't plan to upgrade soon, get the coverage. If you're thinking of upgrading, that's the time to upgrade. (And sell the ibook with enough time left on the clock for the new owner to get applecare.)
As for the rest, transport it in a padded bag. I use a LowePro EX-1300 which has a sleeve in the main compartment that is a perfect fit for the 12" ibook. I mean perfect. It's designed for larger laptops to be slid in side-down but the ibook fits perfectly back (or front)-down. It holds the ibook very snugly. No sliding around. No parts exposed to be scratched by other stuff in the bag. I think I paid $50 for the bag. Maybe less. I'd also recommend a small sleeve or messenger bag for times when you just want the laptop without all of the accessories. Timbuk2 has some nice (if a bit pricey) options. Booqbags has some nice stuff as well.
Cleaning: Don't eat near the laptop. It's one thing if you drip chili in a $20 desktop keyboard. It's another thing entirely if it drips down into your optical drive. Also, the wrist rest areal will get dirty. Doesn't matter how often you wash your hands. It's gonna happen. I scrub at it with glass cleaner once in a while and that keeps it at a tolerable level. Some people use comet/ajax/etc. scouring cleaners. Just be careful that you don't get any cleaning agents inside the ibook. And stick to a damp cloth for the trackpad. On the screen, all I've used is a lint-free cloth moistened with filtered water. If you need more than that to get stuff off your monitor, review the "don't eat near the laptop" section.