When you first start to fiddle with your own environment, much can go wrong! If you do something silly in your .xsession file, your session may not start up successfully.
To get around this problem, login as normal BUT instead of pressing the RETURN key after your password, press the F1 key. This will cause xdm to completely ignore your .xsession file. This will enable you to log in successfully and correct the problems in your .xsession file. Your .xsession file is just a shell script, so once you have fixed the problems you can execute the file like any other.
This is very useful for testing the changes you make as you go, and any error messages will be displayed on the terminal.
As well as the F1 key, there are several other options you can use: