The free space you’ll earn will never expire unless stated otherwise.
1. Register to Dropbox
If you’re new to cloud services, and you’ve never used Dropbox yet, be sure to register and get your initial space of 2GB. Welcome to the world of Dropbox.
2. Getting started guide
Be sure to get through the getting started page, as it is necessary to get free space from two promos offered by Samsung and HTC. I will get to that later.
3. Linking, following and sharing some love
Even if you’re not very sociable, and you hate Facebook and Twitter, get them because it will allow you to earn some space.
Link your Dropbox account to facebook and twitter. This lets you add and collaborate with your contacts on Dropbox.
Don’t forget to follow @dropbox on Twitter.
And finally, share some love on facebook and twitter.
All the steps above will get you 125MB each which is 725MB in total.
4. Get as many referrals as you can
Referring Dropbox to your friends, family or even strangers, if you have some kind of place to promote your referral link, can get you up to 16GB of free space. All they need to do is to install Dropbox on their device.
5. Photo gallery
Dropbox has recently come up with their own photo sharing application called Carousel, both for iOS and Android. You don’t even have to share your photos with them to get free space. Just install the app on your smartphone and you’ll instantly earn 3GB.
6. Mailbox App
Mailbox is a dropbox’s email client for iOS and Android devices. Just install the app, go to the settings, link it with your Dropbox account and enjoy free space of 1GB.
7. Samsung promotion
This one is little tricky because you need to have a Samsung smartphone or tablet with Dropbox preinstalled on the device. If you do, log in to your Dropbox account on your device and go through the tour. Also, be sure to have the "getting started guide" mentioned in step 2 completed. After you’ve successfully done all of this, you will earn 48GB of space that you can enjoy for two years. You’ll even get 3GB for life after the expiration.
If you don’t have Dropbox app preinstalled on your device, don’t worry. If you do all the steps above, you are also eligible to get 48GB, but it will expire one year earlier, and you’ll not get the free 3GB extension.
8. HTC promotion
This is basically the same as the Samsung promotion, but you will "only" get 25GB instead for the same period of 2 years. Log in to your Dropbox account, go through the tour, and have the "getting started guide" completed.
If you own a phone with HTC Sense 4+ (sadly, the promo for Sense 4.0 has already expired) hurry up, because this promotion will end at the end of June 2014. It will also expire at the end of this year for HTC Sense 5.0 users, but you can still use the space for two years.
I have earned much more, but HTC promotion has already expired for me.
If you don’t own a Samsung and/or HTC device, but you want to get that free space so badly, you still can. All you need to do is to find any suitable device, which hasn’t already be used for the promo.
I’ve used testing phones in the company that I’ve worked for. But if that is not an option for you, you can ask your friends and family (or you can just borrow their phones while they’re not looking), or use phones that are part of mobile operators' showrooms. You get an idea.
If you are too lazy to do all the steps by yourself, or for whatever reason it might be, I advise you to find a gig on Fiverr that will hopefully take care of it.