Like me, you might want to use debian testing but occasionally install unstable packages. What happens if you simply put testing and unstable repositories on your sources.list?
If you try that, you will notice that your system will always prefer to install packages from unstable (because apt loves newer versions!). You should, then, put this line on your /etc/apt/apt.conf:
Initially I thought this feature were called pinning, but pinning gives finer, per package control (see links below).
In order to install something from unstable, you must override the default:
aptitude -t sid install some-package
But, when the version of this package on testing pass unstable, it will install from testing again. (This also works with stable/testing; just make sure you default to stable)
Note that AptPreferences on debian wiki will warn against package breakage (but you already knew it, right?). There is a comprehensive tutorial on mixing stable and testing on the wiki of a guy named Freek Dijkstra, also.
Isn’t Debian great? :)