Don't.  Bad for privacy, usually done wrong and hacked, even if done right people often forget or lose the password...

If you still must create a brainwallet, for the love of $deity_or_lack_thereof please make sure you generate the passphrase genuinely randomly - e.g. an Electrum-esque 12 Diceware words chosen with real dice, written down multiple somewheres (perhaps split up) - and use some kind of KDF that requires a substantial amount of time to slow down brute-forcers anyway. 

But, please, just don't even do it in the first place.