As well as using a shared .gitignore file in a directory, I've had a convention of putting my own files I want to ignore in an .ignored directory and having that in a global excludes file.
This works for most situations, but sometimes files need to be in their expected location and not in a sub-directory.
I also have scripts that check for a file in both places before performing an action.
Git, though, has a built-in way to do this.
Any files added to .git/info/exclude will automatically be excluded by Git and because this is in my .git directory, it won't affect anyone else's version.