Dropbox is an online storage platform with a few gigabytes free storage. Your data is stored encrypted by default, but they keep the key and will give it to authorities if asked, so it's only secure to a certain level. By using your own encryption layer, dropbox is encrypting your encrypted data, so they have nothing.
The advantage of dropbox is that it gives you more free storage space if more people sign up through your affiliate link, 500MB per new free user, upto 16 GB in your free account. If you don't want the hassle, you can sign up for a paid account, which is also reasonable priced at about $10 for 50GB.
We will be using encfs, which uses encryption on file level, so you won't have to synchronize the complete container as in other encryption solutions (e.g. truecrypt), but only the changed files.
A dropbox account: sign up here http://www.dropbox.com/
Homebrew (installation manual here)
Install homebrew by using the default installation manual (don't use 'sudo', it isn't necessary!)
Update your packages list: 'brew update'
Install encfs and the fuse layer for mac os x with one command: 'brew install encfs'
encfs ~/Dropbox/secure ~/Documents/secure
Give option 'p' for paranoia, which are the right settings we want.
Choose your password carefully, if you lose it, you can't decrypt your data.
Install the dropbox client software, this will take care of the automatic syncing to the dropbox platform.
Select your ~/Dropbox/secure folder for syncing.
Copy the files you want to backup securely on your dropbox to your ~/Documents/secure folder and it will be automatically encrypted and synced.
One of the other advantages of encfs is that this solution will also work for group shared folders, you just have to share the password and the files are automagically synced.
If you want more paranoia security, you can follow the final steps from this tutorial http://webdiary.com/2011/07/04/securing-dropbox/ to move the encfs configuration file outside of the dropbox syncing folder.