Writing a killer email subject line can make the difference between earning an open and an instant delete. In fact, many users decide whether to open an email based on the subject line alone.

That’s why we’ve put together these tips for writing, testing, and personalizing your subject line.

Writing Your Subject Line

There’s a lot riding on your subject line, so much that staring into your ESP’s blank “subject line” box can almost feel overwhelming. Where do you start?

As you craft your subject line, keep these things in mind:

• Spark their curiosity. Give your audience just enough information to grab their attention, but not so much information that they don’t have to open the email. That being said, you probably want to avoid clickbait or gimmicky subject lines. Opening the curiosity gap can be a powerful way to earn an open, but it’s important to make sure your email’s content lives up to the hype.
• Create a sense of urgency. Nobody wants to miss out on a good deal. That’s why nearly every email in your inbox on Black Friday includes phrases like “6 hours left!” or “Today only!” While it’s important to avoid making empty promises, showing off a time-sensitive deal in your subject line may entice some recipients to click.
• Check your character count. The right subject line length isn’t an exact science, but it’s important to consider what your audience will actually see in their email client. A lengthy subject line will likely be cut off. Whether you go with a single word or a whole sentence, make sure the most important part of your subject line will be visible in the inbox.
• Do the math. When readers have a full inbox, their eyes may gloss over the words in your subject line, but numbers tend to stand out. If your email’s content lends itself to including a number in the subject line, go ahead and try it! What have you got 2 lose?
• Think about your preview text. In many email clients, the preview text appears just below the subject line. Think of this space as a bonus subject line; use it to your advantage! Write email preheader text that complements your subject line and provides extra incentive to click.

“Okay,” you may say, “these tips are nice, but I’m still not sure what to write in my subject lines.”

Unfortunately, there’s no silver bullet for writing subject lines. That’s because no two audiences are alike. Using emojis may send open rates skyrocketing for one retailer while sinking them for another. One audience may enjoy a cute pun on the day’s news, while other audiences will delete it without opening.

That’s why testing is critical.

Testing Your Subject Lines

While A/B testing is important across your email program, it’s especially important for your subject lines. Subject lines are easy to test, and subject line tests can have a significant impact on your email performance.

What can you test in your subject lines? A few ideas:

• Length. Will readers respond to a longer subject line, or should you get straight to the point?
• Tone. Silly? Serious? Concise? Conversational?
• Emojis. Do emojis delight your audience, or direct them to delete?
• Word choice. Do certain words grab reader attention?

Once you’ve decided what to test, you must decide how to test it. Will you split your subject lines 50/50, with half of your audience receiving Subject Line A and the rest receiving Subject Line B? Or will you try out your test on a smaller segment of your audience?

One way to get maximum returns from each send is to test subject lines on a small percentage of your audience, then deploy the winning email to the remainder of your list. For instance, you can send Subject Line A to 10% of your audience, Subject Line B to another 10%, and the higher-performing of the two to the other 80%. With PostUp, you can automate your A/B testing to send the winning subject line automatically.

After you get a handle on your subject line testing strategy, you can turn to more advanced subject line tactics.

Email Subject Line Personalization

While seeing your first name appear in a subject line once earned an instant click, many recipients barely bat an eye at first-name personalization anymore. Everyone puts first names in subject lines.

However, that doesn’t mean you shouldn’t personalize your subject lines. It just means you have to get smarter.

You’ve got user data; use it in your subject lines! Retail email marketers know what products shoppers view, while publishers know which articles readers click. Including behavioral or geographic data in your subject lines may entice email recipients to click.

Of course, to find out for sure, you have to test it! Download our Email Testing Guide to learn how you can implement effective A/B testing in your subject lines and beyond.

Note: Through PostUp’s partnership with Return Path, we have historically offered a Subject Line Optimizer. While Return Path is sunsetting the Subject Line Optimizer feature, we hope that you can use these tips to continue creating strong subject lines.