Well there are a bunch of ways:
There's an invisible bit that's part of the directory structure. You set it, and the file will be invisible to both OS X and OS 9.
Additionally in OS X you can make a file invisible the UNIX way by starting it's name with a period.
If you have the OS X developer tools installed, jump into the terminal and do this:
SetFile -a V thefilename Make it invisible
SetFile -a v thefilename Make it visible
If you don't, download ResEdit (old classic application) from Apple's developer page, and do a "Get File Info" (I think) from within ResEdit, and click on the invisible checkbox.
Beyond that, there are a bunch of little shareware/freeware apps that will do invisible-bit fiddling for you. One product that comes to mind is
Barebones Super Get Info - I think it can do this for you (without the terminal or ResEdit).
- shrill -