The importance of crafting a good subject line really can't be overstated. Just think about your own email habits. How many emails do you get on any given day that you see, and quickly abandon any intention of opening? Probably more than a few. That goes for everyone else too. If you can't hook the audience in with the subject line, the whole campaign is doomed before the message was even able to be received.
As far as length goes, there seems to be a general consensus in the industry that you should keep the length to somewhere in the 40 - 50 character range (or less). A tool like the one at LetterCount.com is a quick and easy way to check the length.
"The best email subject lines are short, descriptive and provide the reader with a reason to explore your message further," says MailChimp. "Splashy or cheesy phrases more often cause your email to be ignored rather than make them stand out."
In a study, the company analyzed the open rates for over 200 million emails. It found that that are three words you should avoid that won't necessarily trigger spam filters, but will negatively impact open rates. These are Help, Percent Off, and Reminder. It also suggests avoiding all capital letters or exclamation marks, adding that questions can often perform better.
You should also avoid repetitive subject lines, it says. If you're sending the same campaign over and over again (such as reminders), open rates will likely decline with each send if they have the same or similar subject line each time.
While Mailchimp does generally concur with the 50-character limit, it found that for campaigns whose subscribes were highly targeted, readers "seemed to appreciate" longer subject lines, so that's something to consider.
It also found that personalization like including a person's first or last name didn't significantly improve open rates, but providing localization like city name, did.
Take a look at the best-performing and worst-performing subject lines from the Mailchimp study (in terms of open rates):
"Your subject line is your first impression on your reader," says Miranda Paquet from Constant Contact. "The words you choose for your subject line can have a big impact on whether or not the hard work you’ve put into your email will pay off. Spending a little extra time on your subject line will help you boost email open rates, avoid the junk folder, and get your message in front of the right people."
While she does advise against spammy use of exclamation marks (specifically using multiple ones), she cites several examples that use one each as effective subject lines, such as this one from Mother Earth Pillows: Essential Oils...Learn what they do!
Her tips also include using: a deadline ("Cruisin’ 4 Critters is August 2nd Register Today!"), a teaser ("Destination Weddings…Want Rock Star Treatment?”), a command ("Join us at the MLH Scholars Luncheon!”), a list ("3 Tools to Simplify Your Marketing”), an announcement (“The winners of the postcard contest are…”), something unique (“Taza Hot Fudge And A Cherry On Top!”), a joke ( “real baseball fans eat burritos”), something unexpected (“Join us for a Bling-ing good time” from The Basketry), or multimedia ("Reel Works Kickstarter – New Video!").
Here's a quick visual on her recommendations.
Last year, WordStream offered 9 approaches to subject line style to increase open rates:
6. Personalized (location or interest targeting)
7. Questions and other punctuation (including "fun symbols")
8. "Missing Out" and other scarcity tactics
Check out the full article for additional insights into each of these styles.
Mequoda offers some additional style ideas, including: Reason Why (“5 Reasons Why You Should..."), Benefit ("Lose Weight While You Sleep"), Testimonial ("Why Arnold Palmer Uses Quaker State Motor Oil"), How To ("How to Make a Fortune with a Foolish Idea"), News ("Federal Home Loan Program Announced"), Fascination (“Discover the Ultimate Options Trading System“), and Seasonal ("Your New Year’s Resolutions for Losing Weight").
Of course you could always go the BuzzFeed route. Here's the three-word phrases it uses for article titles to get the most Facebook shares. Social media is different than email, but some of these are bound to have an impact in the realm of the latter. You know, if you want to go that way.
A recent Business Insider piece rounded up some advice from various professionals with tips including: writing the subject line first (to set the tone), placing the most important words at the beginning, eliminating filler words, being clear about the email's topic, taking search and filter keywords into consideration, indicating if you need a response, personalizing with the person or company's name, creating urgency by limiting the timeframe, and rereading the subject line (nothing worse than a typo in the subject).
In the end, most of the tips in this article should be considered ideas, not gospel. You're going to have to do some testing of your own and getting a feel for your own audience and how they respond to your messages. Then, tweak your strategy accordingly.