Boot and Nuke! http://www.dban.org/
Note: if you don't want to wipe the whole hard drive and reinstall the OS, simply deleting your user profile isn't the most secure option.
There are utilities to shred free disk space: http://www.mireth.com/wt/sxme-shred-freespace-mac.html
, but that is not 100% effective. Your data can still possibly be recovered.
The only 100% sure way that nothing can be recovered later is to physically destroy the hard drive in some way (magnets, drilling, crushing, etc.), but you probably don't want to go that route as you'd need a new hard drive.
I guess it depends on how far you're willing to go. If it were me, I'd use DBAN and just clean install the OS afterward.