When it comes to building an email marketing list, there is no secret formula. But there are steps you can take to increase your opt-in rates and decrease the number of Unsubscribes. What are those steps? Dozens of email marketing pros share their top nine tips for how to build a successful email marketing list.

1. Make it easy for people to sign up. "You'd be amazed how many companies miss out on email signups by not adding an option for people to subscribe on every page of their [site]," says Mike Volpe, CMO, HubSpot, which specializes in inbound marketing. If you want to get people to give you their email address, or sign up for newsletters and promotions, "make it easy and seamless for readers of your content to engage with you."

"Place the subscription form on the most visited pages of your website, including the homepage," says Deborah Hanamura, director of marketing at Metia, a global marketing agency. Also, be sure to "include a tab on Facebook that enables fans to sign up [for emails and promotions]."

As to where to put your email signup, "make sure [your opt-in] form occupies a prominent part of your website, ideally above the fold within the header," says Andrew King, senior strategy consultant at Lyris, a provider of integrated digital marketing software and solutions. And "don't be afraid to [put] your sign-up form in the footer too."

2. Collect email the old-fashioned way, in person. "Collect contact information at business events [conferences, trade shows, mixers]," says Lauren Witte, associate director, marketing & client services, Jackson White, a full service law firm. "Have plenty of your business cards on hand [as] usually if you hand your card to someone you meet, they will give you their card in exchange."

3. Offer people an incentive. "Offer a prize, coupon or free resource [e.g., a white paper] and require an email in order to get your prize," suggests Jim Belosic, CEO, ShortStack, which helps businesses build contests, sweepstakes and data-collection forms on the Web and for mobile. "If you give people something they want, they'll give you what you want."

For example, Bob's Discount Furniture recently offered Facebook fans a chance to win four tickets to an upcoming New York Giants football game if they signed up for their email list.

Similarly, "if you're attending a trade show, or giving a presentation, give away a prize as an incentive for attendees to drop their business cards in a bowl," suggests Witte. "Prizes don't have to be huge. A gift card will do. But keep in mind, the better the prize, the more cards you're likely to receive."

4. Let people know what to expect. "Explain the value [of signing up for your emails]," advises Andy Shore, content & social networking manager, Benchmark Email, a provider of email marketing services. "Let your subscribers know what they're in for. Whether it be deals and promotions or information and education, people are more likely to sign up when they know what to expect."

5. Deliver relevant, customized content -- with a clear, but brief, subject line. "While this may seem obvious, it's more important than ever to deliver content that is tailored to each customer's interests and behavior," says Alyssa Nahatis, director of Deliverability for Adobe Campaign, a provider of marketing campaign solutions. "The digital marketing tools out there today give marketers the ability to customize content with real-time updates for each customer, increasing customer's confidence and trust with the company," she says.

"Don't treat every customer as a nameless, faceless nobody," adds Len Shneyder, director of industry relations at Message Systems, which provides email infrastructure software. To find out what your customers are interested in, "ask them a few questions at the point of sign up to better optimize promotions and offers."

"Sending individualized (including the person's name in the emails) and custom emails based on an individual customer's actions on your website (which you can track by knowing what content they downloaded and what website pages they visited) can help you yield higher email open and click-through rates," says Leah Pope, vice president of marketing, Synthesio, a social intelligence company. Remember, "Nobody wants to receive information that they don't care about," she says. "If you [repeatedly] send [people] something that isn't applicable to them, they will likely unsubscribe or never open any of your emails again."

6. Equally important is to have a strong -- but brief -- subject line. "For subject lines, it's best to tease the content and keep it very short," says Joe LeKostaj, email marketing director, Sprout Social, a social media management and engagement platform. "Depending on your audience, up to 60 percent of opens happen on mobile, and iPhone mail [for example] only displays up to 38 characters in the subject."

Indeed, according to research conducted by market research and consulting firm Chadwick Martin Bailey, some 64 percent of people open an email based on the subject line alone. While according to marketing firm Convince & Convert, 69 percent of people will mark your email as spam if the subject line looks like, well, spam.

7. Humanize your messaging. "Far too many company emails are written like a broadcast message or press release," says Volpe. "Know who humans want to read emails from? Other humans. To that end, write copy that is relatable, approachable and relevant to your audience versus promotional language that sounds impersonal," he advises. "Your audience will thank you."

8. Optimize your emails for mobile viewing. "Include responsive design into your email content to allow for proper viewing as 61 percent of emails are now opened on mobile devices," says Nahatis. "Case studies show that responsive design in emails can increase open rates by 15 to 17 percent and click-through rates by 21 to 24 percent." Moreover, "up to 80 percent of consumers will delete emails that don't render properly when they open them, and 18 percent will actually unsubscribe."

8. Don't inundate people with emails; rather, give them options. "Inundating contacts on your list will leave a bad taste in the recipients' mouths, causing a negative association with your brand," says Steve Clark, vice president and director of Account Services, Garfield Group, an integrated marketing agency. So what's considered "inundating" customers with emails? "It [typically] means sending more than a couple of emails per week, especially during the first two weeks after opt-in," he says. To find the right frequency, "gather analytics to help determine which emails are garnering positive and negative results. [Then adjust] the frequency up or down."

"Brands who email aggressively risk aggravating their subscribers," says Shawn Myers, vice president of marketing, StrongView, a provider of a cross-channel marketing platform used by enterprises. "Actions as simple as giving customers control over frequency can go a long way toward preventing the dreaded unsubscribe. [So] offer the option to opt down or even pause marketing in addition to a full opt out," he suggests. "With these simple practices, organizations can better understand their audience's needs and deliver emails that drive engagement and revenue."

9. Test your email delivery strategy. "Email delivery times, days and promotional windows should all be tested," states Shneyder. "If you routinely send email on Tuesdays, how do you know that Wednesday isn't a better day? Did you test it or just assume it was the day to send out that 20 percent promotion because your competition does the same thing?" If the latter, test to make sure your theory is correct -- by tracking open rates, click-through rates and unsubscribes. And if it isn't, change your strategy.