There's no shortage of choices for cloud storage, but that leads to another problem: how do you decide which services you truly need, and which files to put where? If you've signed up for as many cloud providers as you have files, it's time for an intervention (or at least a moment of clear-headed contemplation).
I'll admit it: I'm an online storage junkie. At one time or another I've synced files to the cloud using Amazon Cloud Drive, Amazon S3, Bitcasa, Box, DollyDrive, Dropbox, Google Drive, iCloud Drive, Microsoft OneDrive, SpiderOak, SugarSync, Wuala, and probably a few others I'm forgetting--not to mention using online backups from Backblaze, CrashPlan, and Mozy, and storing photos with services such as Flickr and SmugMug. Some of these services are free (at least for a limited amount of data) while others are inexpensive, but inexpensive times a dozen or more starts to hurt. Meanwhile, I had the same folders syncing to three or four services simultaneously, which slowed down my Mac, wasted bandwidth, and tested the limits of my ISP's monthly data transfer allowance.
The challenge was what to do about it. "Just pick one!" you may say. Fine, but if I pick Dropbox, then Google Docs can't see my online files. If I pick Google Drive instead, then my iOS apps that support only iCloud won't have access. And so on. Companies like Apple, Google, and Microsoft benefit when you stay within their respective ecosystems, so they tend to make it easier to use their own cloud storage services than those of their competitors. (Microsoft's recent decision to integrate Dropbox support in its Office apps for iOS--supplementing OneDrive--is a welcome exception.)
Even if interoperability weren't a problem, it's not as though these various cloud storage services are otherwise interchangeable. Each one is different when it comes to matters such as privacy and security, saving older versions of files you've since modified or deleted, APIs for integration with third-party products, storage limits, and pricing.
Each person's needs and preferences will vary, but I'd like to offer some tips based on my own experiences in simplifying cloud storage.
Look for broad compatibility
Whatever else you might say about Dropbox, far more apps support it than any other cloud service, particularly on iOS. (It's also quite inexpensive, which doesn't hurt.) Perhaps the scale will tilt toward iCloud Drive at some point, but even if that happens for iOS, Dropbox works on more platforms, including Android and Linux.
So I use Dropbox as my all-purpose cloud storage provider, and probably will for the foreseeable future. If you prefer to use, say, SugarSync for general-purpose cloud storage and all the apps you care about happen to support SugarSync natively, that's terrific--but the odds are against it.
Offers of free (or cheap) storage are tempting, but don't add an account just because you can. Each cloud storage account you use should serve a unique and useful purpose. I cancelled my accounts with several providers because they all duplicated capabilities I already got elsewhere. On the other hand, I keep Google Drive and iCloud Drive, despite their similarities, because each one offers features the other doesn't: namely, integration with the provider's proprietary software.
Don't confuse cloud storage and cloud backups
Cloud backup services such as CrashPlan copy files to distant servers, and let you retrieve those files from another computer or an iOS app. That sounds a lot like cloud storage. On the other hand, Dropbox stores deleted files and old versions for 30 days, or up to a year if you pay extra for Extended Version History. That sounds a lot like cloud backup.
But services that specialize in storage are generally better at keeping your files in sync across devices, while services that specialize in backup are generally better at long-term retention and data restoration (and often have superior encryption, too). Each service meets a different need, so I don't consider cloud storage and cloud backup of a given folder to be redundant. I use both.
Let each service stand alone
Suppose you use iCloud Drive because that's what Keynote works best with, and Google Drive because that's what Google Docs works best with. Fair enough--let each service hold its own documents. If the two sets of files sync independently with your Mac (and in most cases they will), that's even better. But trying to sync all your documents between cloud services is usually a waste of effort (and perhaps, depending on how you do it, a waste of money). That brings me to the next point.
Use aggregators only as needed
Providers such as cloudHQ, Otixo, and ZeroPC let you aggregate cloud storage services--that is, after you connect all your accounts, you can see your documents from every provider in a single view in the Web or an iOS app, drag files from one service to another to copy or move them, and in some cases even sync files between cloud services.
It's a neat trick, and can be a big help if you have files scattered across many services. But although basic plans are free, you may have to pay as much as your cloud storage itself costs for full-featured aggregation. Besides, if you're following the previous tip, you should seldom need to move files from one service to another--and even when you do, you can use your Mac as a conduit and avoid paying for a cloud-to-cloud transfer service.
Go off-cloud for privacy
A handful of cloud storage providers, including SpiderOak and Wuala, offer "zero-knowledge" encryption, which means your data is encrypted in such a way that the provider can't decrypt it without your personal key, even if the government were to demand it. That's great--I'm a huge fan of encryption--but because my favorite iOS apps don't support these services, that severely limits their utility for me.
So, when privacy is important, I either encrypt a file myself before uploading it to Dropbox, or use a "personal cloud" product such as BitTorrent Sync, ownCloud, or Transporter, each of which has unique virtues.
As long as my favorite apps insist on keeping me locked into specific cloud storage services, I won't be able to pick a single provider and stick with it. But I've already reduced my tally significantly, and if more developers make customer-friendly moves like the Microsoft-Dropbox partnership, choosing cloud storage services may be less of a hassle in the future.