You can choose a top-fast mirror,or by your own preference, download its sourceslist (what's sourceslist?) to the directory "/etc/apt/sources.list.d/" or simply overwrite the file "/etc/apt/sources.list".We also strongly recommend you to backed up your original source list file before overwriting.You should change "CODE" bellow to your Ubuntu codename,ie,lucid,marerick,or natty,default is "lucid"(ie,URL without "?CODE" request string, tihs is to compatible with older entry).
Above instructions in command line are also available for advanced users(make sure you have backup original "sources.list" file and changed "CODE" to your Ubuntu release codename.):
sudo wget -O /etc/apt/sources.list http://mirror.ubuntu9.com/topmirror/sourceslist/topfast?CODE
sudo wget -O /etc/apt/sources.list http://mirror.ubuntu9.com/topmirror/sourceslist/topnear?CODE
sudo wget -O /etc/apt/sources.list http://mirror.ubuntu9.com/topmirror/sourceslist/topstable?CODE
sudo wget -O /etc/apt/sources.list.d/0_top_fast.list http://mirror.ubuntu9.com/topmirror/sourceslist/topfast?CODE && wget -O /etc/apt/sources.list.d/1_top_near.list http://mirror.ubuntu9.com/topmirror/sourceslist/topnear?CODE && wget -O /etc/apt/sources.list.d/2_top_stable.list http://mirror.ubuntu9.com/topmirror/sourceslist/topstable?CODE