You never get a second chance to make a good first impression. It's as true in the inbox as it is in life. Subject lines can make or break an email campaign. No matter how valuable or important the content of an email, if your subject line fails to hook the recipient at a glance, your open rate – and therefore the success of your entire campaign – suffers.

So what's an email marketer to do? Here are eight helpful tips for improving your email subject lines and increasing opens:

1. Keep it short. Email subject lines are such valuable real estate that you might be tempted to try and include every detail of your email message. Remember, however, that long subject lines will be truncated by email clients and that short subject lines are more eye-catching for recipients. Aim for 50 characters, or about five to seven words.

2. Utilize personalization. Email subject lines such as "A special offer for Bill Burns" or "Sue, have you seen these deals?" are examples of incorporating personalization into email subject lines. While names are probably the most common use of subject line personalization, other demographic details about your recipients could work well too. For example, a subject line like "Check out these Atlanta restaurant deals" could be very effective at attracting inbox attention by communicating that the information is local to the recipient.

3. Don't be too salesy. Email is an incredibly effective sales tool, but you don't want to sound too salesy right out of the gate. Phrases like "buy now" or words like "free" in your subject line can cause recipients to delete your message in seconds because it sounds too much like a sales pitch. In some cases, overly salesy words and phrases can even cause your message to be flagged as spam.

4. Include a deadline. "Ends Thursday," "Today only," "Final chance" – language like this in an email subject line communicates scarcity. In our always-connected society, there's a huge fear of missing out (FOMO), and incorporating deadlines into your email subject lines is a great way to tap into that mindset. Just make sure that it makes sense to do so in the context of your message; a "limited time offer" that never seems to end will only encourage distrust towards your brand.

5. Be negative. People don't want to do things wrong, and playing to those insecurities can be very effective in email subject lines. You can garner opens by conveying in your subject line that you're going to explain what not to do, thus helping the recipient avoid potential embarrassment. Examples include "Do not do this on Twitter" or "The one thing B2B marketers shouldn't say."

6. Use numbered lists. As in blog posts like this one, numbered lists can be used effectively in email subject lines like "3 winning sales tactics you haven't tried" or "Weeknight dinners: 5 easy recipes." Lists are easy for our brains to process, which makes a subject line that incorporates them more appealing to the recipient and makes them stand out in the inbox.

7. Be on brand. In every email you send, the subject should sound like it's coming from your brand. While humor, for example, is great and can help your message stand out in an increasingly crowded inbox, it can fall flat if it doesn't feel familiar coming from your organization. The same goes for other subject line tones and approaches. If your brand voice is more on the professional side, a casual subject line will seem out of place and off brand, especially to longtime customers and email subscribers.

8. Try split testing. Sometimes the best way to improve email subject lines is to put them to the test in the inbox. Split testing, also known as A/B testing, through ClickDimensions allows you to do exactly that by sending two versions of your email with varying subject lines to determine which version gets the most opens when sent to a set number of your overall recipients. The winning version is then sent to the remainder of your full list, helping to remove some of the guesswork from subject line success.

And don't forget the preheader text. This text comes after your email's sender name and subject, and can help reinforce your subject and further encourage opens.

Happy Marketing!