How to wipe a Mac Mini in preperation to sell it?

I have a Mac Mini that I’ve recently sold. I need to wipe it of all personal data. There’s not a ton - it was never my “real” computer, just something to test Mac stuff with - but there’s some.

If this was a PC, I’d probably just format the drives re-install the OS. But I’m stupid when it comes to Macs, plus Macs tend to have nifty utilities that make stuff like this easy, so I’m asking. What’s the fastest/easiest way to restore this thing to like-new condition, with none of my data on it?

Forgot one key detail: We don’t have the disks for the current OS. It’s been upgraded a couple times via download & install.

Did you agree to sell it with the current OS?
If not, just boot from any bootable OS DVD, go to the Utilities menu, choose Disk Utility, and format the drive. Then you can re-install the OS if you feel like being nice.

If you want to preserve the current OS, it’s a bit more complicated - let me know and I’ll tell you how to do it.
ETA:
Forget all that. Just go to System Preferences->Accounts, make a new Admin account, log into that account, and delete your old account. You may want to “Erase Free space” from Disk Utility when you are done.
Give the buyer the login to the new admin account.

I’ll try the new account thing - that sounds like the ticket. Thanks! I figured there’d be some semi-easy way to do this.