I found this on ArchWiki :http://wiki.archlinux.org/index.php/Xorg
You can edit xorg.conf
To change the keyboard layout, use the XkbLayout option in the keyboard InputDevice section. For example, if you have a keyboard with English layout:
Option "XkbLayout" "us, se"
Option "XkbOptions" "grp:caps_toggle"
This makes your Caps Lock key switch between the different layouts. This is mainly useful if you do not run a Desktop Environment which takes care of keyboard layouts for you.
I also use "setxkbmap en" or "setxkbmap tr" to change the keyboard layout.(en is for english, tr is for turkish... list of keyboard layouts :/usr/share/X11/xkb/rules/xorg.lst)
If you are using KDE or Gnome it is much simpler and you can have an applet near the clock for this purpose. I also experienced that in KDE4 you can assign key combination to switch keyboard layout. For example you can assign windows-key for this operation. it is very useful when you only need to switch to the second layout temporarily. It is like a shift key.