Believe it or not, going negative can also bring an increase in readers. "We're sorry to have to say goodbye" got a 21 percent open rate for one Lyris campaign, while "Take action to keep receiving Intevation Report email tips" and "A better Intevation Report is just one click away" both ranked significantly lower.

Then again, going positive can work, too. Ryan Buchanan, CEO at eROI, found that "Want Fresh Email Campaigns That Increase ROI? Talk To Us Now" brought both significantly higher opens and clicks than the more negative "Is Your Online Marketing Stale? We'll Show You How to Make it Fresh."

In all cases, the best tool for honing in on subject lines that work is testing. Lyris's Peterson often uses what he calls the "wife test." He'll take three subject lines home and show them to his wife, another family member or a friend. The email they open first is the one he'll often use.

For a more scientific approach, Return Path spokesperson Tami Forman recommends using a larger sample size. Choose two subject lines and send each to a portion of the emails in your database. Go with the subject line with the best response rates.

Leah Messinger is a freelance writer.