How can I back up software I have dl'd off synaptic?

Does anyone know how I can back up the software I've downloaded with synaptic onto a disc for safe keeping?

Thank you

you can do in terminal

# dpkg --get-selections > ~/installed.packages.txt

and you have a list ( just a list ) of all packages installed in your system
Then in case of restore you just have to do

# dpkg --set-selections < ~/installed.packages.txt

to select all packages for installing