If your really really paranoid, then you should remove the drive and destroy it, if it's a SSD then that's the only way to truly destroy data. SSD's can't be secure deleted at all despite what software says or does.
If your moderatly paranoid, then learn how to use Lion Recovery (or 10.6 install disk) to 7x erase the entire OS X parittion and reinstall OS X and your files from backup.
If your just wanting to remove traces of your online adult type behavior, embarrasing if discovered, then:
You need something like OnyX's cleaning and maintenance (all!) and CCleaner for OS X, once you have deleted the "caches" reboot the machine.
Mac's have many hidden caches, also you need to reboot the machine to clear the DNS cache too.
Once you do that you need to clean the "free space" on the drive using Apple's Disk Utility using the 7x overwrite feature, takes several hours.. 1x Zero is faster and may be good enough, isn't quite enough for the government spy types and that's how Brian Manning got caught. He used the 35x at first, but that takes several days to complete, it was taking too long so he used the Zero erase (about a hour) and then reinstalled OS X, it wasnt' good enough, the government got the communications with Wikileaks. LOL 😝
If you just want a clean machine for viewing makware laden adult sites, the best way is to use a virtual machine software and a guest OS either Windows or free Linux, then save a "snapshot" of a pristine guest OS version. Revert back to that older snapshot when your done and use the Erase Free Space 7x if your paranoid.
If the law wants to check you out, they will consult the ISP records for the last year to see if you have been doing anything illegal, those records are not erased, I think the government gets a copy of that and keeps it forever. LOL. 😝
You do know your cell phone is tracking you and recording all your behavior on it right?
You need to read Slashdot daily, you learn there is no such thing as online privacy.