Watch dropbox facebook for promotions. They did the dropbox mystery quest last year. Took about 3 hours but I got 30GB of extra storage permanently.
Also, check out BTSync
http://labs.bittorrent.com/experiments/sync.html. I still use dropbox constantly for files I need everywhere all the time but BTSync is awesome for backup and basic sync especially to your phone. I have it set up so that all my movies and pics taken on my phone automatically sync to my home server (unlike dropbox where every device I have would then constantly be updating with all the new huge files). Additionally, I have it set up so that any files I need on my phone just get dropped into a folder on my desktop and boom, they're on my phone but not everywhere else where I don't need them.
EDIT: Oh I forgot, at least on Android you can set it to only sync on wifi and it's super fast! I average 1.8MB/s because it uses the bit torrent protocol.