Here is the command we use. This is a part of our rman backup script. We also have a config script that sets parallelism to 1. One of the other DBA's had better luck with Parallelism set to 4, large_pool_size =256M, dbwr_io_slaves set = 4.

Code:
# storage server was used to store the backup.
run {
   set command id to '${SERVERNAME}';
   configure channel device type disk format '${BACKUPDIR}/%d_%Y_%M_%D_%t_%s_%c_%p';
   backup ${COMPRESSION} database
   tag '${CURR_TIME}'
   format '${BACKUPDIR}/%d_%Y_%M_%D_%t_%s_%c_%p'
   include current controlfile
   plus archivelog;

   configure Controlfile autobackup off;
}