There are 3 ways that I can think of in backing up bbm:
1. Through Desktop Manager - there is an option to either do a full backup or only specific backup. Restoring it goes the same way, you can either do a full restore or just specific database.
2. Through bbm, go to options and backup remotely via yiur registered email addresses.
3. Through bbm, go to options and backup locally. You can even setup bbm to do automatic backup everytime there are changes to your bbm contacts.
For me personally, I prefer the 1st option as it is most accurate and never fails. Had problems in the past with both options 2 and 3.