Oracle - Flash recovery area full

Errors in file /opt/oracle/diag/rdbms/cert/CERT/trace/CERT_m000_11268.trc:
ORA-19815: WARNING: db_recovery_file_dest_size of 53687091200 bytes is 98.96% used, and has 556750848 remaining bytes available.
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You have following choices to free up space from recovery area:
1. Consider changing RMAN RETENTION POLICY. If you are using Data Guard,
   then consider changing RMAN ARCHIVELOG DELETION POLICY.
2. Back up files to tertiary device such as tape using RMAN
   BACKUP RECOVERY AREA command.
3. Add disk space and increase db_recovery_file_dest_size parameter to
   reflect the new space.
4. Delete unnecessary files using RMAN DELETE command. If an operating
   system command was used to delete files, then use RMAN CROSSCHECK and
   DELETE EXPIRED commands.
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Fri Apr 29 22:33:36 2011
Thread 1 cannot allocate new log, sequence 1253

-- The size was increased to 50 Gb.

[oracle@www oracle]$ du -m flash_recovery_area/
1       flash_recovery_area/CERT/archivelog/2011_04_30
168     flash_recovery_area/CERT/archivelog/2011_04_26
62      flash_recovery_area/CERT/archivelog/2011_04_24
50279   flash_recovery_area/CERT/archivelog/2011_04_29
126     flash_recovery_area/CERT/archivelog/2011_04_28
610     flash_recovery_area/CERT/archivelog/2011_04_27
20      flash_recovery_area/CERT/archivelog/2011_04_25
51263   flash_recovery_area/CERT/archivelog
1       flash_recovery_area/CERT/onlinelog
51274   flash_recovery_area/CERT
51274   flash_recovery_area/