I recommend using schroot.
$ sudo apt-get install schroot debootstrap
$ sudo debootstrap squeeze /opt/squeeze
Then in /etc/schroot/schroot.conf add and entry like this:
[squeeze]
type=directory
description=Debian squeeze
directory=/opt/squeeze
users=myusername
root-users=myusername
After that you are ready to install packages in the chroot:
$ schroot -c squeeze -u root
(squeeze)# aptitude
After you install all the packages you need, you can enter the chroot as a normal user (as long as you are listed in the schroot.conf file):
$ schroot -c squeeze
The schroot program is also useful for running newer X11 applications on older desktops, and you only need to pass the environment along as well:
$ schroot -c squeeze -p