1) How often should I back up my data?

This depends largely on how volatile your data is, and how critical it is to the operation of your business.  For most businesses, we recommend that a FULL backup of all data be performed once a week, and in INCREMENTAL backup (just what has changed) be performed on a daily basis.

2) How/where should I store my backup media?

Backup media should always be stored in a different location than the computer it is backing up.  In a small business this should be in an office or storeroom as far away from the server itself.  The media should be kept in a sealed, labeled container, fireproof if possible.  Additionally, one should periodically (once a month) take a copy of the weekly backup to an offsite facility (such as another office, or even your home).