Newest apt package is 6.3.7 but has issues with png resizing.
This may belong on SuperUser, but assuming you mean "6.5.6 or later"...
Ensure that a sources entry for Squeeze (Testing) exists in
deb-src http://http.us.debian.org/debian squeeze main contrib non-free
apt-get update (as root)
apt-get build-dep imagemagick (as root, this installs the build-dependencies)
Change to a directory you'll be using for building.
For the next part you can be a regular user.
apt-get source imagemagick (should automatically pick up the version from Testing, currently 6.6.0)
This will download and unpack the latest source package. CD into the directory created that contains the unpacked source (
imagemagick-18.104.22.168/ as of this writing).
fakeroot debian/rules binary and if all goes well,
In that directory will be a set of imagemagick debs that have only build-dependencies from Lenny (i.e. you just backported imagemagick for your environment).
As root again, run
dpkg -i *.deb.
Enjoy your new packages, but remember to check for security releases periodically and rebuild as needed, as you are now outside of Debian's security release framework. You may want to consider subscribing to the debian-security-announce mailing list.