Taken from here: https://discussions.apple.com/message/20953957#20953957

Step One – Back up your data:

A. If you have any Virtual PCs shut them down. They cannot be in their “fast saved” state. They must be shut down from inside Windows.

B. Clone to an external drive using using Carbon Copy Cloner.

1. Open Carbon Copy Cloner.

2. Select the Source volume from the Select a source drop down menu on the left side.

3. Select the Destination volume from the Select a destination drop down menu on the right

side.

4. Click on the Clone button. If you are prompted about creating a clone of the Recovery HD be

sure to opt for that.

               Destination means a freshly erased external backup drive. Source means the internal

               startup drive.

Step Two – Prepare the machine for the new buyer:

1. De-authorize the computer in iTunes! De-authorize both iTunes and Audible accounts.

2, Remove any Open Firmware passwords or Firmware passwords.

3. Turn the brightness full up and volume nearly so.

4. Turn off File Vault, if enabled.

5. Disable iCloud, if enabled: See What to do with iCloud before selling your computer.

Step Three – Install a fresh OS:

     A. Snow Leopard and earlier versions of OS X

1. Insert the original OS X install CD/DVD that came with your computer.

2. Restart the computer while holding down the C key to boot from the CD/DVD.

3. Select Disk Utility from the Utilities menu; repartition and reformat the internal hard drive.

Optionally, click on the Security button and set the Zero Data option to one-pass.

4. Install OS X.

5. Upon completion DO NOT restart the computer.

6. Shutdown the computer.

B. Lion and Mountain Lion (if pre-installed on the computer at purchase*)

         Note: You will need an active Internet connection. I suggest using Ethernet if possible because

                   it is three times faster than wireless.

1. Restart the computer while holding down the COMMAND and R keys until the Mac OS X

Utilities window appears.

2. Select Disk Utility from the Mac OS X Utilities window and click on the Continue button.

3. After DU loads select your startup volume (usually Macintosh HD) from the left side list. Click

on the Erase tab in the DU main window.

4. Set the format type to Mac OS Extended (Journaled.) Optionally, click on the Security button

and set the Zero Data option to one-pass.

5. Click on the Erase button and wait until the process has completed.

6. Quit DU and return to the Mac OS X Utilities window.

7. Select Reinstall Lion/Mountain Lion and click on the Install button.

8. Upon completion shutdown the computer.

*If your computer came with Lion or Mountain Lion pre-installed then you are entitled to transfer your license once. If you purchased Lion or Mountain Lion from the App Store then you cannot transfer your license to another party. In the case of the latter you should install the original version of OS X that came with your computer. You need to repartition the hard drive as well as reformat it; this will assure that the Recovery HD partition is removed. See Step Three above. You may verify these requirements by reviewing your OS X Software License.