Mac question: Best steps for manually erasing user data if I don't have access to an install CD?

In other words, I can’t boot from anything other than the primary HD. Assume I don’t have any way to obtain an install disc before I need to send off the machine. So I’d like to manually delete as much as possible.

Machine is being shipped to a large and well established cash-for-your-laptop business that I’ve used successfully several times in the past. The business claims to reformat drives before they do anything else, so this is just for a bit of extra safety.