I mainly use Openbox. But after Debian Wheezy installation, X Window System defaulted to LXDE. It's not that much of a hassle to select Openbox from the drop-down menu every time I log on:
However, sometimes I forget to select Openbox, get LXDE, and re-log in with Openbox. This happened quite a few times and I finally decided to change its default X session to Openbox.
The information in this site is the result of my researches in the Internet and of my experiences. It is solely used for my purpose and may not be suitable for others. I will NOT take any responsibilities of end result after following these steps (although I will try to help if you send me your questions/problems).
There are quite a few ways to do this. One way is to edit (or create if it doesn't exist) ~/.xsession or ~/.Xsession.
But I used the update-alternatives command:
$ update-alternatives --config x-session-manager
As shown in above image, select number 2 for Openbox. After logging out, Openbox becomes the default X Window Session!