I use the following command:
All my music, videos etc. are in various folders under 'data1'.
rsync -av --progress --delete /mnt/data1 /media/usba
The backup partition on my external drive is labelled as usba so it mounts automatically at /media/usba.
When first run, this will do a full backup; subsequent runs just apply the necessary changes.
-a = copy file attributes unchanged, as you normally want for backups (more to it than that but that's the basic idea)
-v = verbose = extra output
--progress = progress bars for each file
--delete = any file you delete will also be removed from the backup.
I also backup /home in the same way.
I always keep my external drive disconnected and powered down when not in use so if my PC blows up (like my last one did - flames and all!) there's no chance of it frying the backup drive.
I'm thinking of getting a small fire-proof media safe to keep it in.
If you've got a lot of data and it can't be recreated easily, I'd still do ocassional backups to DVD-R or similar. If your main hard drive gets corrupt and you don't notice, you might copy the corruption to the external backup.
In the past I've used a tool called 'dar' (disk archive) - this can split the backup into DVD size chunks. I think it's in the Ubuntu repositories (I haven't used it recently - I'm not currently practicing what I preach on the DVD front, but I have bought a pack of 50 for when I get round to it!).