Ubuntu

Use 163 mirror for Ubuntu:

Download sources.list file from Ubuntu镜像使用帮助.

Mint

First you need find out your OS release version:

$ cat /etc/issue
Linux Mint 14 Nadia

From Linux Mint Releases we know the corresponding Ubuntu version of Nadia is Quantal, 12.10.

Then add the follow text to the head of /etc/apt/sources.list:

deb mirror://mirrors.ubuntu.com/mirrors.txt quantal main restricted universe multiverse
deb mirror://mirrors.ubuntu.com/mirrors.txt quantal-updates main restricted universe multiverse
deb mirror://mirrors.ubuntu.com/mirrors.txt quantal-backports main restricted universe multiverse
deb mirror://mirrors.ubuntu.com/mirrors.txt quantal-security main restricted universe multiverse

Modify "quantal" to other code name according to your system version.

Or you can specify mirror manually: choose mirror geographically close to your location from Official Archive Mirrors for Ubuntu from Official Archive Mirrors for Ubuntu. I choose USTC Linux User Group.

On this page choose "Quantal 12.10", then add sources.list entries to the head of /etc/apt/sources.list.

Add sources entries of BJTU the same way, so totally it should be:

deb http://mirror.bjtu.edu.cn/ubuntu/ quantal main 
deb-src http://mirror.bjtu.edu.cn/ubuntu/ quantal main
deb http://mirrors.ustc.edu.cn/ubuntu/ quantal main 
deb-src http://mirrors.ustc.edu.cn/ubuntu/ quantal main

Then comment out "deb http://packages.medibuntu.org/ quantal free non-free" in sources.list. It causes errors when upate.

Now run apt-get update, you can find it's much faster than before.