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Can't escape ORA-01555 error!

We are running a RAC implementation with Automatic Undo Management configured. We have also set our UNDO_RETENTION parameter high. We are, however, still getting the dreaded ora-01555 error on a regular basis. There are days when the condition and rate of receiving the errors gets worse as the day goes on. Is there anything specific that can be looked at that would help us greatly diminish our errors (if not eliminate them).
First, I'd check to see if your UNDO_RETENTION is set high enough. It may need to be set high. What constitutes a "high" value? That all depends on your system and its usage. You'll have to query V$UNDOSTAT. Pay particular attention to the SSOLDERRCNT column. If this column contains a non-zero value, which is will since you received the ORA-1555 error, then your UNDO_RETENTION parameter is not set high enough to support your application's operations. You can query other columns from V$UNDOSTAT to get an idea of how much undo space is actually being used. You may be surprised to find that while you are receiving ORA-1555 errors, you are not using a great deal of space in your UNDO tablespace. UNDOBLKS will tell you the number of blocks used in the time slice represented by the row in V$UNDOSTAT. Also check MAXQUERYLEN as it will give you an indicator of how big UNDO_RETENTION needs to be.

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