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Just generate a giant random alphanumber (at least 256 bits to provide maximum entropy) and store it somewhere....
Of course that makes it more secure, but your wallet is no longer a "brain" wallet. If you are going to write down some 256 bit random number, you might as well just write down the private key (encrypted with BIP-38, if you prefer).
The most effective way to make a brain wallet more secure is to use a different hash algorithm. SHA-256 is designed to be fast and is not suitable. Use one specifically designed for hashing passwords, such as bcrypt.