Before selling your Mac, it may be wise to ensure it is completely devoid of any of your secret data. This is easily accomplished by Disk Utility—the only challenge is that you need to start up your Mac from a different volume.

1. Turn on your Mac
2. Put the Mac OS X Install DVD into the DVD drive. The window of this DVD will open automatically. Now double click the icon “Install Mac OS X”
3. Click Continue, accept the license agreement, and finally click Install, Confirm that you want to restart.
4. Your Mac will now boot from the Install DVD.
5. In the first dialog box, you are asked to pick a language
6. Thereafter, the installation process begins. But before we do this, we want to zero the data using Disk Utility.
7. In the menu Tools, select Disk Utility
8. The Installer quits and Disk Utility is launched.
9. In the left-hand pane, select the volume (harddisk partition) you want to erase. Usually it is called “Macintosh HD”
10. In the right-hand pane, first select the tab called Erase
11. Now click the button Security options... and make your choice: if your data is no threat to humanity, it is probably save to pick Zero Out Data. Depending on the size of your hard drive, this fast option will already take several hours, and the other options take 7 or 35 times as long
12. Finally, click Erase.... Note that this action will really erase all data in a way nobody will be able to recover it, no matter how much you are willing to pay for this service.
13. Now you have a shiny empty hard drive. As a nice gesture toward the new owner, you may want to install a fresh copy of Mac OS X. To do this, go to the menu Disk Utility and select Quit, which puts you back into the installer.
14. Now just follow the process, accepting the default values is generally good enough.