The methods that say to remove files and muck about with moving the .git directory are not clean and not using the "git" method of doing something that's should be simple. This is the cleanest method I have found to convert a normal repo into a bare repo.
First clone /path/to/normal/repo into a bare repo called repo.git
git clone --bare /path/to/normal/repo
Next remove the origin that points to /path/to/normal/repo
git remote rm origin
Finally you can remove your original repo. You could rename repo.git to repo at that point, but the standard convention to signify a git repository is something.git, so I'd personally leave it that way.
Once you've done all that, you can clone your new bare repo (which in effect creates a normal repo, and is also how you would convert it from bare to normal)
Of course if you have other upstreams, you'll want to make a note of them, and update your bare repo to include it. But again, it can all be done with the git command. Remember the man pages are your friend.