Mistakes happen--Dropbox learned this the hard way when it accidentally left some user accounts open to the public for about four hours on Tuesday.
This is obviously an inexcusable error, especially for a service that stores users' documents and files on the Internet, but you needn't swear off cloud storage in general--or even Dropbox in particular--because of this regrettable mistake.
Instead, take security into your own hands with these tips to protect your own data on Dropbox.
While it's unlikely that Dropbox will make the same mistake twice, someone could eventually get a hold of your Dropbox password through other means--for example, through all the hacking we've seen recently on other Websites. If you use the same password in a lot of places, your files could be at risk if any one of those sites is compromised. You may want to use a separate password for Dropbox--or, at least, a stronger one than you use for, say, the PlayStation Network. Also, try to avoid using a password that anyone can guess.
Encryption is Key
Let's assume that, even with your iron-clad password skills, someone manages to find a way into your Dropbox account. You can still protect the data itself by using encryption.
The hardcore way to do this is to create an encrypted container within Dropbox by using an encryption utility such as TrueCrypt. Just follow the tutorial for creating new containers (be sure to create it within your Dropbox directory). Make sure the volume is unmounted before uploading the files to Dropbox.
If that sounds a little too crazy, simpler methods are available. Sophos, for instance, offers a free encryption tool, though you'll have to provide the company with some basic information about yourself to start the download. The program creates a special file and handles all encryption and decryption.
Lifehacker has an even simpler method: Create an encrypted .ZIP file for any documents you want to keep safe. WinRAR has a "set password" option under the "advanced" tab when adding files to an archive.
Still worried about intruders potentially deleting your stuff? Consider backing up your data elsewhere. Windows 7 has a built-in backup utility that's actually pretty good. If you're ready to trust the cloud again, you can use an automatic backup service such as SugarSync or Windows Live SkyDrive.
If you're not feeling secure after all of this, you're either beyond help, or you're a secret agent.