You have an awesome email marketing campaign queued up promoting your great offer, and now you're ready to send it to everyone on your subscriber list. You read it over one more time, and nervously hit "send." Hooray, you did it!

Except now you're checking to see how many people opened your email, and the number isn't looking so hot.

What happened?

First off, you're far from being alone. Many of our small business customers at my email marketing company, VerticalResponse, wonder how they can get a better open rate and catch their readers' attention in an increasingly cluttered inbox.

Here's a rundown of six subject line essentials--tried-and-true tactics we've gathered over the years to help increase that open rate and get your readers intrigued enough to click on your email:

1. Include a call-to-action (CTA).

People are driven by action. An email with the subject line, "October Garden Newsletter," isn't nearly as compelling as one with the subject line, "October Garden News + Learn How to Extend Tomato Season."

Similar to the content inside your email, the subject line can be even more effective if you tell your readers what's in store when they open the email or what you want them to ultimately do. CTAs are important because they encourage people to take action, whether it's to sign up for something, redeem an offer or learn something new.

2. Be fun.

Fun, catchy or topical subject lines are often very successful. A subject line that includes an idea, event or story in the news or pop culture may engage your readers quickly--just be sure it relates to the content of your email and can't be misconstrued as being in poor taste.

BarkBox, a company that ships boxes of pet treats and toys to dog owners on a monthly subscription basis, does this really well; sample subject lines include, "13 GIFs of Fuzzy Puppies Who Think They're About That Thug Life" and "There's a Dog in the New Royal Family Portrait." As a dog lover, it would be hard to resist opening these emails.

3. Have a sense of urgency.

Including a limit in which your recipients has to act, like "1 Day Only" or "Only 20 Available," can help motivate them to click on your email. This works particularly well if you sell products or services online and they can buy directly from your site.

4. Get personal.

Try personalizing your subject line with a first name, location, past purchase, etc. to grab attention. Tread carefully, though; we've gotten feedback that while some people appreciate that extra level of customization, others find it too Big Brother-ish and could be turned off.

If you do try personalization, include it within the actual email, too, so that it feels more natural. For example, a recent email from gourmet gifts company Harry & David had the subject line, "Janine, There's Still Time to Order from Your Gift History." Within the email content it greeted me with, "Hi, Janine," and referenced the last purchase I made. For me, this was a convenient reminder from a company I haven't ordered from in awhile.

5. Use an alternative to "free."

There's a whole lot of "free" this or that in promotional emails; it's one of the most commonly used phrases in subject lines. Unfortunately, this means it no longer makes your email stand out, especial now that Gmail filters all promotional emails into a special tab in a user's inbox. Instead of relying on a "free" promotion, try a percentage or dollar-off discount.

6. Offer useful, relevant content.

Unless you specifically state in the sign-up process that you'll only send deals and promotions, try providing helpful, relevant content to your subscribers--tips, tricks and how tos that make their lives easier and/or more meaningful. This is especially important if you aren't a retailer and/or can't regularly offer discounts or promotions.

Tasting Table, a website for foodies, isn't in the business of selling a physical product or service. Instead, their goal is to build a readership. The company hits a home run with their informative emails, like "Say Hello to the Summer's Hottest Pasta Salad" and "Where to Get Your Fried-Chicken Fix," which get their subscribers clicking and reading again and again.

For more tips, check out this guide on writing effective subject lines.

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