The sources.list file is a key factor in adding or upgrading applications to your Ubuntu installation. This is also used by your system for system updates. The file is basically the roadmap for your system to know where it may download programs for installation or upgrade.
Below are the preferred methods for updating/editing your sources.list & repositories:
This is the preferred method of adding repositories via a graphical interface:
This is the preferred method of adding repositories via a command line interface:
Swamped servers and mirrors
After a release, all of the servers & mirrors can be swamped.
If you can wait a day or so they (hopefully) will be back to normal.
Alternately: Open up System|Administration|Software Sources, and then select the "Download from: Other". Click on "Select Best Server". That will go through all of the servers/mirrors and ping each one. It will then come back and tell you which one it got the best response from.