Referrals come to rescue!
I don't how other Dropbox users have done it, I'd tried to invite my friends into this product, but no success at all. This time, however, I followed an instruction from lifehacker, and maximized my Dropbox space in 3 days!
What I have done is basically to run a Google Adwords ad with my referral link as its destination, if there is a person clicked the ad, registered a Dropbox account and then installed the desktop program, I got 500MB free space.
In addition to those keywords mentioned in that lifehacker article, I suggest you to add "cloud" and "cloud storage free" into the keywords list, because "cloud" is such a hot word currently on Internet ;)
I'm enjoying my large free Dropbox right now. But to be honest, this is a hack, and NOT everyone can apply this hack because it requires you to have some FREE Google Adwords credit. In my case, it costed me almost $60 credit to succeed. So please keep this in mind!
I think no one, especially new users, would feel 2GB is attractive in comparison to Google Drive or Amazon Cloud Drive; therefore I really wish Dropbox to enlarge its free plan to 5GB as soon as possible in response to those giant players.
I'll try to ask as many as hotheads I know to read this article!