Brands are always on the hunt for the newest marketing trends to fold into their own strategies. Over the past few years, they've blogged, YouTubed, tweeted, pinned, 'grammed, snapped, and Periscoped to reach a growing population of readers and followers--with some really brilliant results.

But one of the most highly effective content marketing strategies available today is actually pretty old school. It's been around since the '90s (practically retro!).

Yes, I'm talking about email newsletters.

It's easy to shrug off newsletters in favor of newer, shinier marketing tools, but there's one irrefutable reason to use them: They really work. According to a Direct Marketing Association study, every dollar spent on e-mail marketing generates--on average--$38. That's a staggering 3800% ROI.

A newsletter audience is a captive and invested one. Subscribers receive your e-mails because, by definition, they have asked for them in one way or another, a fact reflected in a high (22 percent) average open rate, according to a study conducted by Constant Contact.

Besides that, newsletters are turnkey and affordable to put together, no matter how large your marketing team (you definitely won't need a $30,000-per-post influencer to create and publish it for you).

Whether you want to share your brand stories with customers, stay top-of-mind between purchases, or send your e-commerce business soaring--or a combination of all three--here's how to transform your once-neglected e-mail newsletter into your brand's most valuable marketing tool.

Sell It in the Subject

Think of your subject line as a click-worthy headline on a busy news site: It has to grab attention if you want people to open and read it. According to e-mail marketing tool MailChimp, the subject lines that lead to the highest open rates are typically short (50 or fewer characters), informative, and--when possible--localized to your area. At our editorial marketing company Masthead Media, we've found that it's helpful to A/B test two very different subject lines and determine which is "grabbier" before sending the e-mail to a wider audience.

Provide a Service

Your e-mail newsletter gives you a reason to make contact with your customers between purchasing decisions. In the case of editorial-style newsletters, it's less about selling, and more about providing a service to your customers that ultimately creates positive associations with your company. The thought of paying your cable or gas bills doesn't typically put a smile on your face, but tips on how to save money on your heating from the power company (something New Jersey-based PSE&G does), or a digest of the top movies showing over the weekend from DirecTV, just might.

Resurface Popular Content

What CEO doesn't love getting even more value out of content that's already been paid for? E-mail newsletters are a platform to repurpose the best-performing content from your blog, consumer-relevant information from quarterly reports, or social media posts. Tweak your repurposed content so it fits in your newsletter, and make sure to supplement resurfaced with new content to serve your crossover audiences. If you need some tips on how to do this, I've got you covered.

Keep an Eye on Analytics

There is an element of trial and error to figuring out how many marketing e-mails your customers want to receive. If you see negative feedback in your e-mail analytics--specifically spikes in your unsubscribe or spam rates--pull back on your frequency. To be safe, you can even let your customers control how often they receive emails when they sign up, and allow them the option to change this at any point.

Be Mobile Friendly

The rate of people opening e-mails for the first time from a mobile device has increased by a whopping 180 percent over the past three years. It's now the most popular way people are opening mails. The takeaway here is as important as it is obvious: Optimize your e-mails for mobile viewing or risk certain deletion.