Increase Response to Your E-mail Campaigns

Kirby notes that establishing gender-based segments is an easy way to start testing subject lines.

"Men respond to new products and fresh content, while women often want to hear about special sales and promotions," he says.

Regardless of how you choose to segment your subject line tests, Kirby suggests creating multiple subject lines for each e-mail you plan to send, every time you send. Test these various subject lines on a small portion of your e-mail list, then roll out to your whole e-mail file the subject lines that drove open rates higher than your historic benchmarks. In his experience, optimizing subject lines this way can increase open rates by up to 25 percent.