How to backup my wallet?

First, make sure to update to the latest version of the software. If you created your wallet before mnemonic seeds were implemented (10th of June 2014), please create one with the seed command (open your wallet and type seed). Then write it down somewhere. Second, place wallet.bin.keys* in cold storage (offline). We suggest having two standard-looking, dedicated, write-protected, hardware-encrypted USB keys in different physical locations, with the wallet.bin itself being password-protected. Needless to say, you shall not place the mnemonic seed in the same location as the wallet file (like you should not place you computer password under the keyboard).

* yes, wallet.bin.keys, not wallet.bin. This ain't Bitcoin.