Unless you've been burying your head in the sand the past few years, you may have missed the news about mobile. The "mobile first" revolution has arrived. And it's here to stay.

To boot, some 145 million people in the U.S. owned smartphones (a 60.8 percent mobile market penetration) during the three months ending in August, up 3 percent since May, according to the August 2013 comScore report. Translation: Nearly two out of every three Americans own a smartphone.

Now consider that email is the top activity on smartphones -- ahead of browsing and even Facebook -- and you have a huge opportunity to reach people with your email marketing messages via the devices they are using most often.

However, email marketing hasn't totally caught up with the mobile revolution. The majority of emails are still not optimized for mobile viewing and interaction. Buttons are small. Subscribers are forced to enlarge the screen and move things around to see the email. It's just clunky.

But there is hope. The future is now for mobile-friendly email marketing. Here are seven tips to ensure your next email campaign is optimized for a mobile device.

1. Earn subscribers' trust.
When it comes to mobile, who the email is from becomes that much more important. What's the first thing you see when scanning your inbox? Yup. The "From Name." If subscribers don't recognize who the email is from or don't trust the sender, they are less likely to open the message.

If they don't open your email, the rest of these tips don't even matter. Earning that trust starts well before the first email. It also is not limited to email. Trust can be earned or lost on social media, offline and through other more traditional channels.

Related: How to Write Better Email Subject Lines

2. Really think about the subject line.
Along with the From Name, the subject line is critical. While your audience may not know who you are, a compelling and creative -- or a direct and descriptive -- subject line can be the difference between an open and a delete or ignore.

3. Don't forget about the preheader.
Sometimes called the snippet text, the preheader is the text that's above the header image. On smartphones especially, it's the first bit of text that's viewable.

Instead of something boring like, "To view this email in you browser …" try putting some unique text there. Test clickable calls to action. Maybe even try using some humor.

4. Ensure your call to action is big and obvious.
This is an important step, and not just for mobile-optimized emails. Make sure your call to action is big, bold and obvious.

When it comes to smartphones, real estate is at a premium. Subscribers will not search for your call to action. And sometimes smaller links are more difficult to click on, especially depending on the size of a person's fingers.

Your call to action has to be in their faces. Make it clear, big and simple to click.

Related: 4 Ways to Get Customers to Open Your Emails

5. Consider responsive email design (RED).
Ensuring the user experience is optimized regardless of platform and device is not a new concept on the web. But creating responsive-designed emails is something that is just starting to pick up steam.

This is becoming more important as more people own smartphones and use email as their main "app." Creating a responsive-designed email template is not technically easy to do, but it's something your email service provider or marketing automation vendor should be able to assist you with.

6. Include images.
The majority of email clients on a smart phone -- including the iPhone's native Mail app -- have images enabled by default. Sure, a person can go into the settings and turn them off, but most people don't take this extra step.

So with images on by default, it's important that you think about what imagery you're using in your email marketing messages. Whether your audience is B2B or B2C is irrelevant. Images matter.

So instead of just dropping a random image into your email, consider using something that's linked to the content. Put in a fun image, a different image and an eye-catching image.

7. Be aware of unsubscribe placement.
I believe strongly marketers should learn the love the unsubscribe button. But with mobile devices, it's important to consider where your unsubscribe link is in relation to other links in your email. Too often I've seen the unsubscribe link placed dangerously close to the main call to action. One wrong move and a loyal subscriber has opted out.

Above all, the best advice when it comes to ensuring your emails get opened on a smartphone is to test -- test all of the tips mentioned above. After all, your audience is not my audience. Best practices are those that are best for your subscribers.

Related: Why Marketers Should Stop Worrying and Learn to Love the Unsubscribe Button