A disclaimer/warning is needed: UNLESS you KNOW what you're doing, you probably are better off building packages for each of the supported Debian distros.
I know it sounds cheesy, but I couldn't find a better name.
At my workplace we are packaging software to work for as many distros as possible at one time. As a consequence the .deb packages we produce should work fine on all Debian distros: oldstable, stable, testing, sid.
To make sure we achieve that, we are building them in an oldstable chroot. OTOH, using pbuilder is very demanding on I/O, especially when entering the chroot and when exiting it. That slows down a lot the build system and is time to change that.
One reliable alternative is to use cowbuilder (part of cowdancer package). What is interesting is that cowbuilder relies on the existance of /usr/bin/cow-shell inside the chroot, so the cowdancer package needs to be installed both in the chroot and the host machine.
But cowdancer was not a part of sarge, so, if you try to create a new sarge cow directory with cowbuilder, it will fail at the end saying it didn't found the cowdancer package. Also trying to login via cowbuilder --login will complain it didn't found /usr/bin/cow-shell.
The fix for this is simple, once you figured it out and saw that cowdancer is available in sarge as a backport:
cowbuilder --create --distribution sarge --basepath /var/cache/pbuilder/sarge.cow --othermirror 'deb http://www.backports.org/debian sarge-backports main'You may prefer to be sure that the only thing from backports is cowdancer and use a more hands-on approach. Also, this will eliminate the need to correct later the apt sources to exclude backports.
Write the same command as before, but omit everything starting with --othermirror. Then install by hand the package in the chroot.
twix:/home/eddy# cowbuilder --create --distribution sarge --basepath /var/cache/pbuilder/sarge.cow
twix:/home/eddy# chroot /var/cache/pbuilder/sarge.cow
twix:/# wget http://ftp.de.debian.org/backports.org/pool/main/c/cowdancer/cowdancer_0.23~bpo.1_i386.deb
twix:/# dpkg -i cowdancer_0.23~bpo.1_i386.deb
twix:/# rm cowdancer_0.23~bpo.1_i386.deb
twix:/home/eddy# cowbuilder --update
Since after the bootstrap all .deb packages will be in the apt cache in the chroot, you might want to save some disk space:
twix:/home/eddy# cowbuilder --login --save-after-login
twix:/# aptitude clean
If anything is broken somehow, missing or whatnot, cowbuilder --login --save-after-login is your friend.
 the initial bootstrap does not pull from backport anything else but cowdancer
 since you want to compile for plain sarge, not sarge+backports ;-)