How can I set up a USB HDD to extend the life of my SD Card and maximize the performance of my Raspberry Pi?
If you're going to have a HDD that's always attached to the Pi, then you can mount the sections of your filesystem that incur the largest number of read/writes directly from it.
These directories are probably the culprits:
/home/ /var/ /tmp/
You are able to mount partitions on your external hard drive to these directories automatically at boot. Let's say your HDD is
/dev/sdb, and it has four partitions. You can append your
/etc/fstab to look something like this:
/dev/sdb1 /var ext4 defaults 0 1 /dev/sdb2 /home ext4 defaults 0 1 /dev/sdb3 /tmp ext4 defaults 0 1 /dev/sdb4 none swap sw 0 0
I've also included a swap partition. Though you might want to research how effective swap can be over USB. I really wouldn't expect much from it.
More information about swap in this question: How to set up swap space?