Your kidding right? Wax?
I don't use my toolbox everyday, but I know it'd still be a pain if I needed to run in the garage to grab a pair of pliers real quick or something like that.
If I'm painting near the box I throw a blanket over it though. Dust I just blow off with the air gun when I clean the place or maybe wipe it down with a rag if I'm feeling really ambitious.
I wax my box (that sounds dirty). I also wipe my tools down with alcohol before I put them away. Once a week, I wipe down the top with glass cleaner. That's probably why my 7 year old C-Pro box looks nicer than 90% of the 1-2 year old snap-on boxes at work.
A. I paid enough for it.
B. I'll have it for years and want to take care of it.
C. It looks like hell to customers when a mechanic has some ratty old beat to hell toolbox. If he doesn't take care of his own stuff, why would he take care of their car/plane/motorcycle/whatever?
There are two guys on my crew with Snap-On boxes 3-4 years newer than mine. They don't take care of them, and they're dirty, scratched, cut up tops, etc. Do their boxes get noticed? Nope. The customers come talk to me about mine, and half the time mention how ratty the other ones look. While this may not translate to how they work, the perception is most definitely there on the customer's part.
The same basic ideas apply to my box at home.