How to start applications on specific desktops
I like to have Firefox (Iceweasel) always on Desktop 2 and Thunderbird (Icedove) always on Desktop 3. This rules can easily be defined in the <applications> section of ~/.config/openbox/lxde-rc.xml:
<application class="Firefox-bin"> <desktop>2</desktop> </application> <application class="Icedove-bin"> <desktop>3</desktop> </application>
Now when I start Firefox or Thunderbird/Icedove it always shows up on the right desktop!
Note: I use LXDE with Debian Lenny; you would have to replace "Icedove-bin" in the above XML config with the appropriate WM_CLASS of your mail program (use xprop for this).