What files and directories should I exclude from a backup?

From Backup Central

On UNIX systems, you should exclude /proc, which is your system's memory represented as a filesystem. You may also want to exclude files such as core files, the cdrom partition, and the /tmp partition. Remember that the /usr/openv/netbackup/logs/ directory can grow at an enormous rate if bpdbm is logging.

On both NT and Unix systems, you should also exclude database files that you are backing up by other means (e.g. Oracle, SQL Server, etc.).