When I need to do this, I use the duplicate feature. It's very straight forward and creates the new DB with a new DBID automatically.

Rman> connect target login/password@target catalog rmanlogin/pwd@rmanDB auxiliary /

#aux DB should be in nomount state
RUN
{
# the DUPLICATE command uses an automatic disk channel
set until time "to_date('May 20 2010 10:50:00','Mon DD YYYY HH24:MI:SS')";
SET NEWNAME FOR DATAFILE 1 TO '/export/home/oracle/AUX/system01.dbf';
SET NEWNAME FOR DATAFILE 2 TO '/export/home/oracle/AUX/undotbs01.dbf';
SET NEWNAME FOR DATAFILE 3 TO '/export/home/oracle/AUX/example01.dbf';
SET NEWNAME FOR DATAFILE 4 TO '/export/home/oracle/AUX/example02.dbf';
SET NEWNAME FOR DATAFILE 5 TO '/export/home/oracle/AUX/tools01.dbf';
SET NEWNAME FOR DATAFILE 6 TO '/export/home/oracle/AUX/users01.dbf';
SET NEWNAME FOR DATAFILE 7 TO '/export/home/oracle/AUX/logmnrts.dbf';
SET NEWNAME FOR DATAFILE 8 TO '/export/home/oracle/AUX/marius01.dbf';
DUPLICATE TARGET DATABASE TO AUX
LOGFILE
GROUP 1 ('/export/home/oracle/AUX/redo01a.log',
'/export/home/oracle/AUX/redo01b.log') SIZE 10M REUSE,
GROUP 2 ('/export/home/oracle/AUX/redo02a.log',
'/export/home/oracle/AUX/redo02b.log') SIZE 10M REUSE;
}

Then sit back and watch it go.

Jim