1.  Don't trick a user into signing up for your email.  If they select the box to be included in your email list, they are much more likely to want to receive the kind of information you are sending. 

2.  Segment your audience.  Divide your audience in a way that makes sense.  You probably wouldn't send the same marketing email to mothers with young children as you would to business professionals. Teen marketing should look different than cardholders of AARP.  By segmenting your audience you can then make sure your message is relevant to them.

3.  Use honest subject lines.  Any trickery in your subject line will not only hurt your relationship with your audience, it may cause the content to be less relevant. If you need to use trickery to get people to open your emails, you probably aren't providing them with enough value. Send relevant, valuable information and people will want to open the email.

4.  Stay true to your message.  While it may be tempting to follow a certain fad or popular meme in hopes of increasing your opens, staying true to your message will be more effective.  Most people sign up for a company newsletter to get discounts or to learn more about the industry, so going off topic will risk losing the interest of your audience.

5.  Let recipients choose the type of emails they want to receive.  If you send out multiple types of emails (newsletter, promotions, partner products, etc.), let your recipients choose from the different types of emails to ensure they stay subscribed.  For example, they may only want to receive your newsletter because you include useful industry information.  Or they may only be interested in receiving coupons / promotional offers. By segmenting the type of content you send, it will keep your audience listening to your message.

6.  Use your analytics. Be sure to review the results of every campaign.  If a particular campaign had a high unsubscribe rate, review the subject line and content to see what you might have done wrong.

The longer you can keep users subscribed and reading your emails, the more likely they are to become (or remain) a customer of your business. Email is a great constant reminder that your business is there, ready to serve them. Just be sure to keep them paying attention by sending useful, relevant content.

Have other ideas about how to keep users subscribed?  Share them in the comments below.

 Lindsey Winsemius is the Vice President of Communications for ApogeeINVENT, and is the social media voice and blogger for the company. Follow her on Twitter or connect with her on Google+