1. Export Private Key
2. Encrypt file using AESCrypt
3. Save encrypted file to `Dropbox
You can leave out step 2 because its already encrypted with AES, just make sure you choose a strong password.
I personally also would not want to rely on Dropbox, I like to keep backups of my files on my own storage devices and store them offline.
You can also backup the wallet file itself instead of the key export, this is even easier:
* copy the *.wallet and the *.info file to one or more external USB sticks
* store them in safe places (at least 2 sticks and not stored in the same house)