I like to create a partition on my backup drive to be the same size as the OS X partition and use superduper to make an exact bootable image of the original. Just make sure you use GUID partitioning if it is an Intel Mac.
I then use the remainder for storing extra temp video and photo files.
If you use bootcamp you will need a Windows partition and winclone to back that up as well.
I have 4 external drives and rotate them offsite to a safe deposite box as well. I never want to lose any data!