You could change the owner to system. This way every change you make to the folder, you have to put in your password just like the actual system folder.
Hit Get Info and change owner to system. You will likely have to relaunch the Finder. Also give your user account read-only permissions.
Basically set permissions to:
Owner: System
Access: Read & Write
Group: <your user name>
Access: Read-only
Others: Read-only or No Access
Having to authorize every change can be a nuisance though. If you back the folder up regularly, you should be ok.
Do a File > Get Info and then check the "Locked" option. This will prevent the file from being deleted even if you put it in the Trash and try to empty it.
Comments
Hit Get Info and change owner to system. You will likely have to relaunch the Finder. Also give your user account read-only permissions.
Basically set permissions to:
Owner: System
Access: Read & Write
Group: <your user name>
Access: Read-only
Others: Read-only or No Access
Having to authorize every change can be a nuisance though. If you back the folder up regularly, you should be ok.
start using Time Machine
Quoted for emphasis. Seriously. Buy a backup drive NOW if you don't have one.