Oracle recommends that you use RMAN to back up files managed by Oracle ASM. The procedures for using RMAN are documented in Oracle Database 2 Day DBA.
You can also back up your Oracle ASM managed files by performing a files related operation using the following procedures:
Create an alias.
Create a directory.
Delete the alias and directory.
Rename the alias and directory.
For information about migrating data with RMAN, see Performing Oracle ASM Data Migration with RMAN.
For additional information about RMAN, refer to the Oracle Database Backup and Recovery User's Guide.