This is the /etc/apt/sources.list I'm using on most of my Debian boxes.
As I often also add sources from both table and testing (and sometimes unstable and even experimental), I also have APT pinning set up in /etc/apt/preferences.
Adding the third party repository keys
I trust these third party sources, therefore I add their keys to my local APT key ring:
debian-multimedia.rg
# apt-get install deb-multimedia-keyring
opera.com
# wget -O - http://deb.opera.com/archive.key | apt-key add -
skype.com
# apt-key adv --keyserver pgp.mit.edu --recv-keys 0xd66b746e
percona.com
# apt-key adv --keyserver keys.gnupg.net --recv-keys 1C4CBDCDCD2EFD2A
