Well, there's apt-pinning to do this - however, sarge (testing) is using the compiler GCC 3.3-1 while woody uses 2.95, see here
. You might run into several problems here.
Thus, you better use backports to get new versions of certain software packages, see apt-get.org
or google. For example, apt-get.org has no results on "mailscanner" but "+mailscanner +backport" in google shows some hits, one of it is here
Usually, you have to add repository mentioned to your /etc/apt/sources.list and run "apt-get update". Running "apt-get install <your_software>" then updates the package and its dependencies. The disadvantage of so doing is that "apt-get upgrade" might install newer versions you didn't wanted, so be careful what you write into your sources.list
To avoid this, you can download each package from the backport adresses and their dependencies to build a local repository. This is a little bit of work but worth the effort sometimes.
If you can't find a backport of your wanted software, you can also build one yourself. This is easier than it sounds in most cases.