I cannot stress enough how important it is to back up your code and your database on a regular basis. If you don't own the code then remind your clients to back up their code.

Technology is not perfect and things do happen. Make sure you have a backup plan that will fit your needs in case of the unfortunate issues that may arise. If you host your site with an ISP find out what their backup schedule is and how much it costs to recover data. Do not reply on these backups but make sure it is there. Store your backups on a different server or at an off site location.