mine is:

Year / month + main topic / file name

ie: 2007 / 04 great barrier reef / hamilton island 1234.RAW
2006 / 07 east coast tasmania / maria island 4567.RAW

I keep the original raw file number in all future versions of the image as that way i make sure i locate the right raw file in need. Using 04 (rather than april) means i can easily sort them into month order. edited versions are kept in a sub folder, inside the main one, these subfolders are called "TIF files" and "JPG files" and retain the naming convension.

I make sure i tag all my images when they are uploaded from the memory card, this makes finding shots a LOT easier. I use Abobe Lightroom to upload, tag, and add copyright data all at the same time.

If you are fastidious about Tagging images really well, in theory you could have all your files in one big folder, but i wouldn't recommend that.