Although I believe that email open rates aren't the most important statistic, it's still worth investing time in raising the percentage of subscribers who open your email.
Here are some things to test:
Subject line: Is the subject line really relevant to your reader? Does it indicate that the information inside will help him do his job better or make her life better? Will he consider it a must-read? Or, is the subject line intriguing enough that she will open the email out of curiosity?
Your reputation: Are you somebody who consistently delivers value? Was the last newsletter you sent worth reading? (After all, you probably only get one chance with a new subscriber.) Can you be trusted to give independent advice as an expert, not a salesperson? Do you make occasional sales pitches or is every email a sales pitch?
Your list: Consider who you are sending your email to. As you sending emails to just one list - with all sorts of prospects - or are you segmenting your list according to your subscribers' interests? Have people signed up to receive your information, or are you spamming them? (This last one makes the biggest difference - sending unsolicited emails will kill you.)
Here's the bottom line: Is your email newsletter really adding value to your readers' lives? Is it worth the click?
After all, a click is something you have to earn.