Here are just a few of things I keep in my hook caddy:
Good quality hooks, duh, right? No, seriously, having a good quality hook can really save your fingers, hands and wrists after all that repetitive motion. Invest in good hooks. This is the one I use the most, Crochet Lite Crochet Hooks. They are very comfortable in the hand, plus the hook lights up! How cool is that. They also make great gifts for the "hookers" in your life. They come in all sizes, make sure to keep some extra batteries as well. Knit Lite Battery Replacement
I also keep a nail file in my caddy. It NEVER fails, I get all cozied up with my project, yarn set, hook in hand,, and SNAG. Oops, theres a hang nail. I dont really want to put everything down, get up and go searching for a nail file. I have learned just to keep one in the caddy at all times.
Another odd thing for my caddy is Dental Floss. It is very handy to use to pull yarn through beads and buttons.
Yarn needles are another necessity for me. I hate trying to weave in ends with a hook, plus it gives a much more finished appearance. Also a requirement for joining squares neatly.
Add scissors for cutting yarn, and don't forget a tape measure to check your gauge.