Another way, sneaky, though hardly the safest (but it's easy), is to hide the folder or document inside an application package.
Right click an app, open it with Show Package Contents and toss in the document or folder.
I created a TextEdit document with the secret but exciting sentence "She sold sea shells down by the sea shore," named the document Sea Shells and put it in the iCal app to see whether Spotlight or the Command/Apple-F search could see it. Neither could (and I re-indexed Spotlight, to make sure).
But the freeware app EasyFind found it when the optional Package Contents was turned on and it searched for the title, Sea Shells.
EasyFind also found it when it searched for the phrase She sold, after I turned on the optional File Contents as well as Package Contents. But if you don't have EasyFind, that doesn't matter. Even if you do, who would think to search for a folder or document in application packages?
It would be safer, though, to use an app that isn't likely to be updated or replaced entirely by a newer version.