Files should be split up logically by purpose. This applies to code, headers, resources, and all other files that can be split up. See the section on comments.
The file names should be as meaningful as possible.
All code/declarations/etc. should be relevant to the declared purpose of the file.
Header files declaring functions, if not tied to one specific source file, must have comments stating which function is in which source file -- and, as usual, these comments must be accurate!