If you want to install the same packages that already have been installed on another system the following small script (export_list.py) could be of help:
It prints the names of all packages that have been installed explicitly and not as a dependency. It seems to be more elegant than just copying the dpkg selections.
Save the script to export_list.py and run it on the original system to create the list:
Copy the list file to the new system and run the following command: