Several months ago I created an article with 5 APT (Advanced Packaging Tool) tips for both Debian and Ubuntu available here. APT is the package manager in Debian and Debian-based distributions, like Ubuntu. Here's part two of that article, with 5 more tips and tricks for APT.
Notice: I specified which commands need root privileges. In Debian, type su, followed by your root password, while in Ubuntu just precede each command which needs root privileges with sudo. Notice that by default there is no feedback when entering the password, so you won't be able to see it, but it goes in. Just press Enter when you're done.
Add repositories from a CD or DVD
If you need an offline repository for packages, you can download all the ISO images for your distribution, eventually burn them on CDs/DVDs and add them manually to the /etc/apt/sources.list file using apt-cdrom (as root):
Enter the CD/DVD in the drive and let APT scan for packages.
Another great solution is to not even burn the ISO images to CD, just mount them and add them to your sources.list this way. Here's what you have to do: