How to add the RPM Fusion and livna repositories to Fedora?

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http://unix.stackexchange.com – RPM Fusion and Livna.org are common third party package repositories for Fedora. You need them if you want to install media players, codecs and/or DVD playback libraries that are not part of the primary Fedora repository because of assumed issues like distribution licensing or similar. Thus my question how to enable them in Fedora (>= 17)? (HowTos)