I don't believed that you can do an online database backup yourself for CRM Online. Microsoft will back up the databases automatically. If you decided to move from online to on-premise, you may ask Microsoft to backup your database and send it to you so you can use during your on-premise environment setup.
If you are a CRM Online customer, open a support ticket with them and they might able to provide you a better answer.
Darren Liu | 刘嘉鸿 | MS CRM MVP | English Blog: http://msdynamicscrm-e.blogspot.com | Chinese Blog: http://liudarren.spaces.live.com