If you're using RMAN to take your backups, and are running Oracle 10g, do yourself a favor and enable block change tracking now. Prior to doing so, my level 1 incremental backups were scanning my entire database (~830 GB) and took over 5 hours. With BCT, level 1 incremental backups only look at around 45 GB (according to v$rman_backup_job_details) and often take less than 30 minutes.
The one caveat is to watch for the overhead incurred, but we haven't seen any noticeable performance hit.
Of course my level 0 still takes over 9 hours, but I'm hoping to (re)implement incremental merge and only take one level 0 to start and never look back.