This is really easy to do, just do signup method, referrals and method to get coupon codes.
Good luck with sales though maybe someone will fall for this and buy it.
True anyone can do this, there's a few hurdles you have to get over but it is very possible to do it by hand.
The signup part is easy, no tricks there - not even a captcha (which is key for my script-solution).
Then you'll need the dropbox client, a fresh mac address and cleared dropbox settings.
This is because, before the referral space is awarded, the client of the new user needs to be linked to the new account.
Starting the dropbox client, you're asked to visit a http-link that will connect your client with the account (the link uses the mac address, and possibly the wan IP) - you visit the link and you're done.
You'll need to do this 32 times, clearing dropbox settings and switching mac address - which is fine to do on your normal machine, but i'd recommend virtual machine clones.
I found 32 times to be like doing it 1 mio. times. Maybe that's just me. Also, coupon codes?