Example - stable
The sources.list for using stable reads --
deb http://http.us.debian.org/debian stable main contrib non-free deb http://non-us.debian.org/debian-non-US stable/non-US main contrib non-free deb http://security.debian.org stable/updates main contrib non-free
Example - testing
You need to include the stable lines, and then add three lines with a s/stable/testing/g, giving the file:
deb http://http.us.debian.org/debian stable main contrib non-free deb http://non-us.debian.org/debian-non-US stable/non-US main contrib non-free deb http://security.debian.org stable/updates main contrib non-free deb http://http.us.debian.org/debian testing main contrib non-free deb http://non-us.debian.org/debian-non-US testing/non-US main contrib non-free deb http://security.debian.org testing/updates main contrib non-free
Source: http://www.mayin.org/ajayshah/COMPUTING/debian-principles.html
On the testing, notice that the sources are accessing stable and testing.
You can find your sources in /etc/apt/sources.list