Create the new partition and write the changes to disk. Note the device that is associated with the partition: it will be something like /dev/sda5.
Fire up a terminal, then execute:
ls -l /dev/disk/by-uuid/
Locate the device that you created. The output will look like this:
lrwxrwxrwx 1 root root 10 2010-10-10 12:00 35746711-2b3a-44bd-b75e-e9782243dbce -> ../../sda5
You need the device's UUID - in my case, that is 35746711-2b3a-44bd-b75e-e9782243dbce. Take note of this value.
Then, mount the root partition (using normal Gnome/KDE/similar interface) and edit the file named /etc/fstab inside that partition. The command is:
sudo gedit /media/YOUR_PARTITION/etc/fstab
To edit that file and copy your old files, follow the instructions here. You will need to change the UUID and the filesystem type, of course, from what is in that post.