1. Install from debian 5 (net install) – only the minimum packages (without desktop)

2. After reboot change /etc/apt/sources.list to point to squeeze repositories.

(I added contrib and non-free repositories)
my example:
deb http://ftp.si.debian.org/debian/ squeeze main contrib non-free
deb-src http://ftp.si.debian.org/debian/ squeeze main contrib non-free

deb http://security.debian.org/ squeeze/updates main contrib non-free
deb-src http://security.debian.org/ squeeze/updates main contrib non-free

3. # aptitude update && aptitude -y install apt aptitude dpkg
4. # aptitude safe-upgrade
5. # shutdown -r now
7. # aptitude dist-upgrade
8. # upgrade-grub