Did you know that email marketing is the best acquisition tactic for subscription sales, as reported by more than 150 subscription site executives in our 2013 Online Subscription Benchmark Report?Yet, while there’s a plethora of tips out there for email marketers, few are tailored for subscription content, i.e., marketers whose content is the product.But if you’re trying to sell your premium content to loyal free users, here are three great tips to optimize your campaigns:

1. Make sure to include a table of contents in your newsletter. Email marketing expert Jeanne Jennings says that table of contents definitely improve email performance. Just make sure you get those click-through stats by linking your TOC headlines to your website, not to article summaries further down in the email.
2. Use a double opt-in confirmation for your in-house lists, especially those acquired through your site. Double opt-ins have significantly lower unsubscribe rates. Part two of this tip: Use a lightbox overlay to capture those opt-ins.
3. For newsletters, use multiple headlines separated by commas or a pipe symbol (|) as your subject line, and watch your open and click-through rates grow! (Caveat: Audiences differ, so make sure to test!)

Of course, that’s just the beginning! It’s important to know what keywords work, when to use in-house vs. third-party lists, and how to adhere to CAN-SPAM laws (i.e., marketing messages need unsubscribe links but transactional messages do not), and you can discover 15 best practices in Subscription Site Insider‘s How-To on Email Marketing Basics for Subscription Content.