Boot from the installer disc. Erase/format the drive. Select write zeros once in security options to make sure somebody cannot easily recover your files (unless you really don't care). Writing once is usually sufficient and will still take a long time (hours if you have a very large drive in there; days if you selected the top level option).
Re-install the operating system after wiping. Make sure you give the buyer the original discs that came with the computer (they do you no good and you can't sell them) and any installer discs for any upgraded operating system and software you leave on the computer (required by license).
Kappy's extensive hard drive formatting post - http://discussions.apple.com/thread.jspa?messageID=8340551 - note that instructions at end are for zeroing data.
Pondini: Formatting, Partitioning, Verifying, and Repairing Disks... - http://web.me.com/pondini/AppleTips/DU.html
I think the only thing I'd trust would be to completely fill the drive with junk - update dmgs, shareware downloads and youtube nonsense.
I'm probably wrong but I do have a very vague memory that zero formatting a drive wasn't full proof?
Opps I should've said - fill the drive with junk by first putting it in another machine or external drive case
When you format a drive you are basically doing the equivalent of removing the table of contents from a book that had no specific page order. The content is still on there but harder to access. Inexpensive, common data recovery tools will look at the drive and try to reconstruct the contents and probably do a reasonable job.
Wiping a drive with writing zeros once is like taking an eraser to the above book written in pencil. Not only is everything out of order but use a good eraser and a standard data recovery tool can't read it. Maybe a stronger tool can read some of the residual magnetic field. Somebody would have to want that data awfully badly and be willing to pay a lot (thousands of dollars) and then spend time piecing together all the bits and pieces.
Write zeros 7 times and the residual field is even weaker. Try to read a piece of paper where somebody has gone over it 7 times with an eraser. Maybe if you have great eyesight and something to bring out the molecules of ink residue on the page you might make out parts of sentences. We're talking major professional data recovery here.
If the drive was used to store the credit card numbers of all VISA owners in the USA then take it out back and smash it to pieces with a sledge hammer. If it had your iTunes collection and family photos go with wiping zeros once for nominal security. Frankly if you are really paranoid about this it is cheaper to just put in a replacement drive and junk the old one.