"Humans have a natural desire for closure– we don't like having gaps in our knowledge. You can leverage this desire for closure by leaving your subject line open-ended so subscribers will be curious, like a cliffhanger that can only be satisfied by opening the email."

That's from OptinMonster, which updated their email subject line guide last week. I particularly like that idea about curiosity. I read somewhere else that messages with a blank subject line were opened 8% more often than those with no subject lines. I guess that's the ultimate in curiosity—or keeping it short.

Speaking of which, there doesn't seem to be any statistical link between subject line length and open rate. However, if you have a lot of subscribers reading your campaigns on mobile devices, shorter may be better.

Here are some more tips for effective subject lines.

Pain points. "If you really understand your buyer persona, you should know their biggest pain points," OptinMonster writes. "Use those pain points to get subscribers to open your emails by solving that problem for them." A couple of their examples:

Uber: "Since we can't all win the lottery..."

Evernote: "Stop wasting time on mindless work."

Duolingo: "Learn a language with only 5 minutes per day."

Retargeting. These are sent to subscribers when they fail to complete an action or a step in your sales funnel (cart abandonment or failure to purchase after a free trial). These emails serve to bring your subscribers back to your sales process—such as "Hey, forget something? Here's 20% off." Or "The price just dropped for something in your cart."

Corrections. Of course, we don't want to make mistakes, but they happen, and an interesting lesson came out of one recently. Simone Bunsen, marketing and events manager for Chief Executive Group and a SIPA Rising Star, told me that they sent the wrong information in a marketing email. "We immediately realized our mistake and decided to send out a corrected version of the email, apologizing for the mistake." Their subject line started with "Oops..." and then offered an apology with $100 off for registration for an upcoming event. It was incredibly successful. What a smart way to seize an opportunity.

Directness over trendiness. "Sometimes, it's better to be direct and descriptive than trendy. Seasonal slogans such as 'Fall into savings' or 'Sizzling summer bargains' are popular, but don't offer a specific hook. Instead, try to communicate the benefits of your promotions, or call attention to specific deals." I've also talked about using company anniversaries and milestones which seem more specific and celebratory than the seasons.

Personalize. Email subject lines that are personalized by including a name boost open rates by 10-14% across industries.

Be original. Writes OptinMonster: "Keep in mind that being original is the key to sustainable success with your email subject lines. The truth is, subscribers get bored easily. If you want to engage your first-time openers and long-term inactive subscribers, you don't want them to read your subject line and think, 'There's that weekly newsletter again that I always ignore.' You'll need to keep mixing things up over the long haul." Try twisting around a cliché or using a different take—can be as simple as he or she becomes she or he.

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Ronn Levine began his career as a reporter for The Washington Post and has won numerous writing and publications awards since. Most recently, he spent 12 years at the Newspaper Association of America covering a variety of topics before joining SIPA in 2009 and SIIA in 2013 as editorial director…