1) Compression: this will easy shave off 25% of the file size. My 2.6 G blockchain file uncompressed became 1.9 G compressed with zip.
2) Split it up in multiple files. If I stop using bitcoin for a couple of months and then start using it again I might still have a large chunk of the blockchain left on my computer. I really only need the new block. The current mainline has a new way of storing blockfiles (.bincoin/blocks/blk000?.dat instead of .bitcoin/blk000?.dat) each with size 128 MB (instead of the much larger .bitcoin/blk000?.dat files). These smaller, 128 MB, files would be nice to have available via a torrent, making it possible to limit the download to only the latest blocks.
im sry but zip compression totally sucks...
read this fora nice list: http://www.maximumcompression.com/index.html