This is how I've configured my APT system to prioritise stable packages, but also allow me to install packages from the testing and unstable pools if I really want to. This is what's called pinning in Debian lingo.
$ cat /etc/apt/preferences
Package: *
Pin: release a=stable
Pin-Priority: 700
Package: *
Pin: release a=testing
Pin-Priority: 650
Package: *
Pin: release a=unstable
Pin-Priority: 600
With this in place, whenever I do apt-get install, it will pick
packages from the stable package pool. If I want to install something
from e.g. the testing pool, I add the -t parameter:
# apt-get install -t testing iceweasel
