Keep it in your brain. Bitshares X is a brain wallet.the term brain wallet is missleading ..
it is not brain wallet as in NXT where the private key is derived from the passphrase.
The private key usually is random (although you can generate one via a brainwallet)
As far as I know it's deterministic and based around a based around some words. Just memorize those words and you should be able to reconstruct the wallet or am I wrong?
I guess this is something which needs to be clarified in the documents.
In NXT the (one and only) private key is generated from the 'passphrase' and thus you can use a memorizable 'brain wallet' (set of common words). If your brainwallet is stolen, so are your funds!
In bitsharesX the main (wallet) private key is randomly chosen and encrypted by the passphrase .. if your wallet is stolen, the funds are safe because encrypted.
Within the wallet you have multiple accounts (account names) and each account has several deposits (effectively, due to TITAN, independent addresses with each having its own private keys)
however .. ALL keys that are needed to either access your deposits, your account, or market orders are DERIVED DETERMINISTICALLY from either the wallet private key, or the account private key (for account related accounts: withdraws, trades etc...)
So: all keys (except the main wallet private key) can be regenerated from the one wallet private key (unless you important some privkeys by hand)
you can surely try to memorize the private key of your wallet. It has the form
private key: 48bbc81a0e210326c7b86d455255eda91990c84b74ebe12ec21930c8181ceca8
or as private key WIF format: 5JNKTjT8FdygfjivjFxdRxGzvd1q4KMFrvHk81hcKukZUEi84Ke