Here is how to track testing, but update some packages from unstable
http://www.debian.org/doc/manuals/reference/ch02.en.html#_tracking_literal_testing_literal_with_some_packages_from_literal_unstable_literal
http://www.debian.org/doc/manuals/reference/ch02.en.html#_tracking_literal_testing_literal_with_some_packages_from_literal_unstable_literal