Your customers, and their information, is your most business asset. You need to have a realistic plan for backing up this information, and for recovering this information in the event of a loss.
Information about your customers can be stored in several files on different systems. Make sure that you back up each system and file in a way that you can recover and rebuild.
Some thoughts about setting the right balance of what get backed up, and how frequently:
How often do you update information about a customer, does this change depending upon how active the customer is ? Consider frequently backing up only the information that has changed, and having a longer period between full backups.
Can you afford to be out of contact with your customers for one day, one week, or one month ? It may be possible to set up a recovery process that quickly recovers key information and then brings back the rest over a short period of time.
Wikipedia has some great background reading on planning for disaster recovery.
As always, if you have any questions or comments, please do let me know. I will be happy to help.