For a less ad hoc approach to reducing storage costs, I suggest looking into the ZFS filesystem. Compression is completely transparent in ZFS. Once you enable compression in ZFS, all of your files will automatically be compressed when written, and decompressed when read.

I am currently managing a Postgres cluster with a petabyte of data in it. We found ZFS to be a great way to reduce overall storage costs. We just switched our machines to machines running ZFS, and we were suddenly using 1/3rd the amount of disk space. Although it took us a while to learn all of the gotchas of ZFS, it wound up saving us a huge amount of $$$.

(As I understand it, ZFS would not have helped in Flickr's case. Since JPEGs are already compressed, ZFS would not have provided any benefit. Flickr was able to save storage by using an ad hoc compression algorithm.)