Direct-mail marketing experts have known this for decades: A call to action as a P.S. of a letter increases reader response.

You can adapt this strategy for your emails. Place a P.S. at the bottom of your email signature—or between your name and contact information with space above and below the P.S.

Use it to ask for referrals, tout a service, or ask for another type of response. You can take another tip from the direct-mail pros by tracking responses and periodically changing your P.S. message.