If you want to exclude your staging site from Google using robots.txt without running the risk to forget deleting the file once you go live, name the file robots.exclude.txt instead.

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In your Apache Vhost config, rewrite requests for the staging server only:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^staging\.project\.com$
RewriteRule ^robots\.txt$ /robots.exclude.txt

Your robots.exclude.txt looks like this:

# This file is returned for /robots.txt on staging servers
User-agent: *
Disallow: /

Important Note: If your setup is incorrect and /robots.txt is not accessible, it means there is no protection at all!