4 Ways to Get Customers to Open Your EmailsIf no one is opening your emails, it's impossible for them to click through to your website or make a purchase. Follow these tips for better email marketing.

If you want to be asuccessful email marketer, it's essential that subscribers open your messages. After all, if your audience isn't opening your emails, it's impossible for them to take action, such as clicking through to your website or making a purchase.

But how do you keep them opening youremailson a regular basis? There are four primary ways: solve a problem, save them money, make them smarter or entertain them.

Here are some tips for crafting these types of successful marketing emails:

1. Solve a problem.
If you knew that an email marketing newsletter would help solve a problem you were having, would you subscribe to it? If the sender set expectations up front and promised that every email would lead you closer to solving that problem, you'd open those messages, wouldn't you?

One example of a company that does this right is Quibb, a professional news site that allows people to share what they're reading for work. It helps its subscribers solve their problems by digesting news and allowing readers to quickly catch up on what's relevant in their industry. Quibb's problem-solving approach translates into an average open rate on its daily digest email thatranges between 50 percent and 70 percent. That's significantly higher than the average marketing email open rate (in North America)of 25.6 percent, according to the Direct Marketing Association.

Related:3 Rules You Must Break to Expand Your Email Marketing List

2. Save them money.
Groupon and other daily deal emails have proliferated by offering subscribers the opportunity to save money. Sure, you have to spend money to save, but it can be enticing to get 50 percent off a dinner at a restaurant you've always wanted to try or 40 percent off the oil change you've been putting off for months.

Similarly, business-to-consumer marketers often put words such as "free," "save," "sale" or "free shipping" in their subject lines. Many people -- my wife included -- save such emails in their inbox for the next time they're shopping in a store or online. Then, they search their inbox for the promotional offer.

For your own marketing emails, test different types of offers. Sometimes free shipping can be more effective than a percentage discount. Other times, a dollar amount savings may work best. Try a subject line split test to see what resonates most with your audience.

3. Make them smarter.
Some of us embrace the "always be learning" motto. To hone our skills, we read business or trade publications, or we take courses. Many marketers exploit this desire to become smarter by sending emails that promise just that.

Related:The 4 Essential Parts of an Email

An example is social media expert Chris Brogan's weekly Sunday email. Brogan shares what's on his mind with the goal of making his subscribers smarter. In a recent email with the subject line "The Sidewalk, The Storefront and the Back Room," Brogan talked about "touch points of opportunity" -- essentially, how your potential customers can find you. His open rates are often higher than 40 percent, and many of his weekly words of wisdom are shared on social networking sites, helping him attract more potential customers to his email list.

If your emails tend to be focused on selling, try mixing it up next time. Don't sell, just inform.

4. Entertain them.
Some emails include an entertainment component to try to increase readership and sales. For example, MarketingProfsincluded a fun video in a blog post and emaillast fall to promote its annual B2B Forum. While it's uncertain exactly how effective the video was in terms of open rates, MarketingProfs did sell more forum passes after the email went out.

I've been doing this withmy weekly emailfor several months. The video is consistently the most clicked -- and shared -- link in the entire email, often resulting in more email sign ups.

It's possible to craft an email that both entertains and saves subscribers money, or one that can both make people smarter and save them time. But most emails focus on only one of the four themes.

Take a look at your recent email marketing messages. Can you identify which of the four reasons your subscribers are reading your emails? If your answer is "none of the above," you might want to reconsider your approach.

Related:The Best and Worst Times to Send an Email (Infographic)