What's a good way to wipe a hard drive for resale?

Solution 1:

As current hard drive-oriented techniques for file sanitization are ineffective on SSDs, I recommend to encrypt the whole hard drive using Filevault 2 (best if you did this before you put your data onto it).
(This is only possible in Lion, Filevault 1 in older versions of MacOS will only encrypt your home folder.)

This way you will not erase your data, but make them inaccessible without the password.

Your buyer will then have to reformat and reinstall MacOS for his use.

Solution 2:

From the Apple Support pages Securely erasing a disk

To securely erase a disk or partition: In Disk Utility, select the disk or partition to erase, and then click Erase.

• Specify a format, and enter a name for the disk.

• Click Security Options and choose to write over the data once, 7 times, or 35 times. Click OK.

• Click Erase.

Writing data over the data 7 times meets the U.S. Department of Defense (DoD) 5220-22-M standard for securely erasing magnetic media.