How to start OS X <code>Single User Mode</code> 

When your Mac is up and running in multi-user mode (the normal operating mode) it is running at level 1.

Some operations even root can't do at level 1, such as turn off the System Immutable flag. You must run at level 0. Switching into single user mode will allow one to run at level 0 and thus change the System Immutable flag.
  (e.g for trash problem - <code>cd /Users/your-name-here/.Trash</code>  then <code>chflags -R noschg *</code>).

1. From start-up, hold down Command-S (or Option-shift-L for read-only mode).

2. Enter ">console" instead of your UserName in the login window.

3. Close all applications open a <code>Terminal</code> session and issue the
 command: 
  <code>sudo shutdown +0</code>

When you enter single user mode, type the command: <code>whoami</code>

If the answer is not "root" type the command: <code>su</code>

When finished, type the command <code>logout</code> or <code>reboot</code>.



When you are finished, type control-d twice when you exit.

http://www.osxfaq.com/Tutorials/LearningCenter/HowTo/Trash/page2.ws