You can use a plugin like backwpup that lets you:
- Store backup to Folder
- Store backup to FTP Server
- Store backup to Amazon S3
- Store backup to Google Storage
- Store backup to Microsoft Azure (Blob)
- Store backup to RackSpaceCloud
- Store backup to DropBox (free)
- Store backup to SugarSync (free)
and manage the number of backup copies to store which means for example you set the number to be 5 and when you reach backup number 6 the first back will be deleted for you.