How do I prepare a Mac for sale?

Hi guys,
I have been using my Mac Mini as my main computer for the last 3 years but now I need to upgrade. because of this, I also need to sell this Mac Mini on eBay but I'm a bit worried that a buyer would be able to find my passwords to bank details and such like. So, is there a way to sanitise my computer to totally remove any such info?
Cheers,
Steve.

If your Mac mini has an optical drive, then it came with a DVD (presumably with Snow Leopard on it). You can boot from that and erase the hard drive (pass of zeros, don't go overboard; a single pass is sufficient).

You can then install a fresh copy of Snow Leopard on it, and when it reboots and starts the registration process, just let it roll until it stops to ask for information. Wait a little bit so that the hard drive is not actively doing anything, and then power it off by holding the power button for four seconds.

At this point it is restored to "factory" condition and ready for a new owner. They can update (if needed) to 10.6.8 through software update, then jump to Mavericks, download apps and all the rest of it.

Thanks, Chas. I'll see if I can find the discs that came with the Mac when I got it.