网上流传的export http_proxy=http://yourproxyaddress:proxyport是行不通的，虽然改了之后wget一类的可以用。当然去改.bashrc也不会有效果。
真正的解决方法如下：
新建文件 apt_proxy_conf(文件名任意)
内容如下：
Acquire::http::proxy "http://127.0.0.1:8000/"; Acquire::ftp::proxy "ftp://127.0.0.1:8000/"; Acquire::https::proxy "https://127.0.0.1:8000/";
往后就可以在apt-get之后加上 -c apt_proxy_conf 来执行。
更改源的步骤如下：
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
sudo vi /etc/apt/sources.list
sudo apt-get update
附录为Ubuntu 12.04在sohu的mirror（/etc/apt/source.list）：
deb http://mirrors.sohu.com/ubuntu/ precise main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-security main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-security main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
