An interesting subject.
Full disclosure: I work for the top remote backup company in Denmark.
I can see that you all are doing a lot more than most people.
I strongly recommend that you use some sort of cloud storage for your backups. preferably two different providers.
With that out of the way, here is what I DON'T recommend:
Flash memory of any kind!!! They will fail! Use for transport and convenience only.
The old harddrive you have in the drawer from your old computer. Its just waiting to die on you, and will do so even if it isn't plugged in.
If you are going to use a harddrive in your backup rotation, then do what SensualPoet does and buy a new set of drives once a year.
CD/DVD is an attractive option when you look at the price, but reliability over time is appalling. If you resort to this medium, do remember to read out the data every 3 years and burn them to a new disc (don't use the cheap disks).
The solution to bitrot hasn't been found yet, and for long term storage paper is still king.
I hope this was of some use
-Klaus
Edit: With me working for a backup company like that, it is obvious that I never do backup of my own computer