From ArchWiki

This is an example of how to use Include command to configure Pacman servers.

[edit] Introduction

You can make your own Pacman server file using Include in /etc/pacman.conf .
I'm going to show an example using the file /etc/pacman.d/servers.conf but you can call it whatever you want.

[edit] Changes to /etc/pacman.conf

/etc/pacman.conf is Pacmans main configuration file and he reads it every time he is invoked.
This is an example of /etc/pacman.conf that contains an Include at the bottom.

#
# /etc/pacman.conf
#
[options]
LogFile     = /var/log/pacman.log
NoUpgrade   = etc/passwd etc/group etc/shadow etc/sudoers
NoUpgrade   = etc/fstab etc/raidtab etc/ld.so.conf
NoUpgrade   = etc/rc.conf etc/rc.local etc/pacman.conf
NoUpgrade   = etc/modprobe.conf etc/modules.conf
NoUpgrade   = etc/lilo.conf boot/grub/menu.lst
HoldPkg     = pacman glibc
IgnorePkg   = kernel26
#XferCommand = /usr/bin/wget --passive-ftp -c -O %o %u
#
# This line includes another file
Include = /etc/pacman.d/servers.conf
#

[edit] Server file

Pacman starts from the top when it is syncing mirrors so it's important that you sort the with that order in mind.
In this case Chemnitz is used first, Antesis second and so forth.
If you want to use testing repository, it should be placed first in order.
It's also possible to use a local repository, eg at the bottom.
An up to date server list can be found here

#
# /etc/pacman.d/servers.conf
# My main Pacman server configuration file.
#
#[testing]
#Server = ftp://ftp.tu-chemnitz.de/pub/linux/sunsite.unc-mirror/distributions/archlinux/testing/os/i686
#Server = http://archlinux.antesis.org/testing/os/i686
#Server = ftp://ftp.ibiblio.org/pub/linux/distributions/archlinux/testing/os/i686

[core]
Server = ftp://ftp.tu-chemnitz.de/pub/linux/sunsite.unc-mirror/distributions/archlinux/core/os/i686
Server = http://archlinux.antesis.org/core/os/i686
Server = ftp://ftp.ibiblio.org/pub/linux/distributions/archlinux/core/os/i686

[extra]
Server = ftp://ftp.tu-chemnitz.de/pub/linux/sunsite.unc-mirror/distributions/archlinux/extra/os/i686
Server = http://archlinux.antesis.org/extra/os/i686
Server = ftp://ftp.ibiblio.org/pub/linux/distributions/archlinux/extra/os/i686

[community]
Server = ftp://ftp.tu-chemnitz.de/pub/linux/sunsite.unc-mirror/distributions/archlinux/community/os/i686
Server = http://archlinux.antesis.org/community/os/i686
Server = ftp://ftp.ibiblio.org/pub/linux/distributions/archlinux/community/os/i686

#[custom]
#Server = file:///home/custompkgs