In the previous versions of Mac OS X you can securely delete files. I found this very handy for certain financial documents that I wanted to remove without the risk of having anything left behind should my Mac become compromised or stolen. Normally this would be accessed from Finder > Securely Empty Trash or right clicking on the Trash Dock icon and selecting Secure Empty Trash. This has been removed in the latest version of Mac OS due to a security venerability, but there is a method of doing this with Terminal.
This is an advance tip, so you need to have a little experience with Terminal. To do this open Terminal from Applications > Utilities. Type the following
Now drag and drop the file or folder you want to securely delete. Then press enter.
Terminal will do its thing and will securely delete the file.
This process will take a while so I recommend you delete folders at a time rather than each individual file. Alternatively you can move all of the files you want to delete into one folder and then delete that.
I hope that Apple will bring back the secure delete as this method is a little long and annoying.