Step Four: Nuke it from orbit, it's the only way to be sure
Here, I'm talking about erasing your hard drive. You never know where it's going to end up, so you want to make darn sure none of your personal information is retrievable.
The best way to do this is to boot from a Mac OS X installer DVD, go to the Utilities menu when the installer comes up, and select Disk Utility. Select the drive (usually given the horribly unoriginal name of Macintosh HD), and then click on the Erase tab of Disk Utility.
Before you click the Erase button, I suggest that you click the nearby Security Options button. Here you have a couple of options, each of which is progressively more secure than the previous. My personal choice is always the 7-pass erase, which (as you can see in the screenshot below) meets the US Department of Defense 5220-22M standard for securely erasing magnetic media by erasing the drive index files and writing over the data seven times. Note that overwriting data actually hasn't been in the standard since 2007 -- the DOD now requires degaussing or physical destruction of drives -- so Apple is a bit behind the times. The overwriting process takes a while -- it often takes eight or more hours to do a 7-pass erase on a 250 GB drive. If you're really paranoid and have a lot of time, why not do a 35-pass erase?
