using gitignore to ignore (but not delete) files

I have a tmp directory in my git repo I'd like to still exist, but be ignored. I added it to .gitignore, but git status still tells me about changes to files in that directory. I tried git rm -r --cached, but that removes it from the remote repo. How can I stop tracking changes to this directory, but still allow it to exist? I also need to do this for 1 file, but changes to that also show up in git status after .gitignoreing them. What should I do?