Find one of the many "$100 of Adword credit free" coupons around, create an ad with your Dropbox referral link, sit back and watch your storage limit increase a couple times per hour.
I wouldn't recommend this unless you have specifically received one of these coupons at your email address. I used someone else's and my account was permanently suspended. Only after some negotiation was I able to make Google reverse the "permanent" suspension (a rarity, from what I have seen).
It wasn't hard to find one that I received myself. I've got coupons in magazines, hostgator, domain resellers... it shouldn't be too hard to come across one legitimately.
