If you want to have the latest software for Debian, but you only want to upgrade some specific software, and not all your debian just follow this instructions

First, change to root

$su -

Then edit with your favorite text editor, (I use vi)

#vi /etc/apt/get/sources.lst

Mine looks this way, maybe yours look different.

#deb file:///cdrom/ sarge main

deb cdrom:[Debian GNU/Linux 3.1 r3 _Sarge_ - Official i386 \
Binary-1 (20060904)]/ unstable contrib main

deb http://mirrors.kernel.org/debian/ stable main
deb-src http://mirrors.kernel.org/debian/ stable main

deb http://security.debian.org/ stable/updates main contrib

# Backports
deb http://www.backports.org/debian/ sarge-backports main

That is all, but if you want to use backports only for selected packages, and not for all.

Edit or create the file /etc/apt/preferences

#vi /etc/apt/preferences
Explanation: see http://www.argon.org/~roderick/apt-pinning.html
Package: *
Pin: release o=Debian,a=stable
Pin-Priority: 900

Package: *
Pin: release a=sarge-backports
Pin-Priority: 200

Package: *
Pin: release o=Debian
Pin-Priority: -1 

Now, if you want a package from backports just do this.

#apt-get -t sarge-backports install foo

You can read all the explanation about this at: http://www.argon.org/~roderick/apt-pinning.html