I have about 200 gigs of space solely for keeping the anime on my hard drives, but eventually, even those fill up. After a series has sat completed on my hard drives for a while (a little over a year), I burn them to DVDs. I'd like to use dual-layer, but they're just not economical. A spindel of 50 DVD+Rs on sale at Staples for $19.95 are hard to beat.
DVDs are permanent (for the most part) and clear up space in the most external way possible. If you want to watch the series again, just copy them back to the HD.
Ideally, what I would want is a Removable HD rack. Then you can have huge amounts of anime in a swappable set of 250 gig drives. Label them by year...*drools*