How do I make apt-get install from Debian's unstable repository?
have a look at apt-pinning. Be very careful or you will find yourself running unstable entirely. On a production system, you probably do not want to do that.
Don't mix stable and unstable packages. Not on a production system, not on your home system, perhaps not even on a random testbed you set up just to see how badly it'd explode.
Look at backports.debian.org for backports to stable of specific packages you may want.
If those are still not as updated as you want, set up a test system and see if the 'testing' tree is stable enough for your purposes and has the software you want.
If that still isn't enough, even running a full unstable tree is almost certainly less likely to break catastrophically than a hybrid stable/unstable hodgepodge.
Finally, as an absolute last resort, you might want to try downloading the source packages, installing
devscripts and then building the packages yourself in the stable system.
Judicious use of apt-get -b source. This allows to handle dependencies more flexibly and does not need dangerous deb lines (just deb-src lines) in sources.list.