.gitignore is not working

Sometimes I want to ignore a file, I add it to .gitignore and the file doesn't disappear from my change list. I want to remember the following, since I always end up reading the same answer to this question.

Even if you haven't tracked the files so far, git seems to be able to "know" about them even after you add them to .gitignore.

NOTE : First commit your current changes, or you will lose them.
Then run the following commands from the top folder of your git repo:

git rm -r --cached .
git add .
git commit -m "fixed untracked files"