By far the most common is to use this as your brainwallet:
private key = Sha256(passphrase)
This is also what brainwallet.org did.
That's insecure. For new brainwallet applications:
1) Consider if a brainwallet is truly necessary. If not, use more secure means. If it is, go to step 2.
2) Use a "slower" key stretching function with multiple rounds (to prevent brute force attacks), and use a long and secure pass
phrase.