Keyboard navigation of Mac OS X Finder window's search options toolbar
The Spotlight search box is the one in the top-right of the menu bar (commandspace). The Find search box is the one I think you're talking about, within a Finder window.

To get focus back to the search field, hit controltab or commandf.

Another option is found in System Prefs > Keyboard > Shortcuts. There's a radio button near the bottom labeled "Full Keyboard Access". "All Controls" will let you tab through the other search fields without modifier keys.

That said, in my testing, there're some flaws. Some of the fields aren't selectable, and if you try to select one of those and then hit commandf, you can no longer tab through the other fields anymore—you have to open a new Finder window and start over.