I think a basic over view of cloning is helpful. I have spent the better part of the last year scripting rman backups and a cloning script, so I feel well versed in the subject area.

In order to clone to another database you need an rman backup. It can be a hot or warm backup, or an incremental hot or warm backup, You should also have whatever archive logs are available from around the time of the backup. You need to create an init file for the new clone. Since the sid name and directory structure will likely be different, you can tell Oracle how to rename the data and log files by putting the following two lines in your init file for the new database.

db_file_name_convert=(/${SOURCE_SID}/,/${TARGET_SID}/)
log_file_name_convert=(/${SOURCE_SID}/,/${TARGET_SID}/)

You need to create the admin, data, archive directories. You need a backup of the control file from the source database, which rman refers to as the target database. You need to copy the control file to the places mentioned in the new init file and do a startup mount. rman can't restore a database unless the database you are restoring to is in mount mode. It helps if you have an rman repository, because rman will then know exactly where to look for the backup.

rman << EOF
connect catalog $DBPASS;
connect target sys/${SYSPW}@${SOURCE_SID}
connect auxiliary /
spool log to /oracle/log/clone_${SOURCE_SID}_${ORACLE_SID}.log;
run {
set command id to 'clone';
duplicate target database to "${TARGET_SID}";
}
EOF

This script will restore the files, do the necessary recovery, rename the database and do the open database resetlogs. You can also doin all of that manually without having the repository. Just point rman to the right backup.

RUN {
ALLOCATE CHANNEL C1 DEVICE TYPE
DISK format '/oracle/rman/sidname/bck_20100415';
RESTORE DATABASE;
RECOVER DATABASE;
}

If you go the manual route rman will leave the database in mounted mode. You will need to rename the database yourself. This is the kind of thing where you need to practice in advance, and make sure that your backup process is giving you everything you need for recovery and cloning.

Good luck.