Brain wallets are by convention uncompressed. While I can now (with help of your answer) create a compressed key from a passphrase and import that into my wallet, I might have a hard time to reconstruct it later for another wallet when I don't have my scripts at hand.
The Web site at bitaddress.org (or the equivalent script you download to your own computer) can do the complete conversion for you.
1. Under the BrainWallet tab, enter a phrase. I will use "correct mule battery staple".
2. The BrainWallet page generates an (uncompressed) bitcoin address of 1Aop6KxiZLccPPPjqmfZHYgnmCKuhiVq57, which we will ignore.
3. The BrainWallet page generates a private key of 5JRks4Vf268r9cuCKiod2iFz1VcSpawX5m6T3PKSA1v7cRqfZZD, which we copy over to the WalletDetails tab.
4. The WalletDetails page generates a compressed bitcoin address of
1JMsC6fCtYWkTjPPdDrYX3we2aBrewuEM3, which we can give to our payors.
5. The WalletDetails page also generates a bitcoin private key, flagged as "compressed", of
KyvGbxRUoofdw3TNydWn2Z78dBHSy2odn1d3wXWN2o3SAtccFNJL, which we can import into a wallet that supports the compressed keys.
You are now a good citizen. As always, test your own generated BrainWallet (receiving and sending) on a very small amount of BTC, before you trust anything substantial to a BrainWallet or any other manually concocted wallet.