Fifty characters could be all that stands between you and success in your next email campaign.

Fifty characters is all the space you have in a typical subject line to catch your reader's eye and entice him to open your email and take the action you want.

How could something so small make or break an email's success? Because many recipients use the subject line to decide whether to open or delete an email.

Subject lines are tricky devils, however. A good one can get your email opened in a flash, while a bad one could spell oblivion in the trash or junk file.

Because so much is riding on your subject line, we came up with 15 rules for crafting a good one. Be sure to review them before you send your next email campaign.

15 Rules to Write the Subject Line Right

Rule 1: Read the newspaper.

If you want to write a better subject line, pick up your local paper. The headline usually highlights a story's most important fact in a limited space. A subject line, in turn, should clearly state what your reader can expect from your email, what's in it for them or what you want them to do as a result of the email. However, there isn't enough space to do all of them all the time. Look at the newspaper headline to see how it interplays with the story.

Rule 2: There is no sure-fire formula.

What works in one campaign might bomb with the next. A discount offer should be worded differently from an upsell, and both are different from a breaking-news announcement.

Even if you are sending out emails to promote similar campaigns, you shouldn't recycle a subject line from a past campaign. You need to stand out each time, yet be familiar to the reader, too. How do you find out what works best? See Rule 3.

Rule 3: Test, test, test.

Test continually to determine trends and styles that appear to work. Pretest if you can. Add a day to your campaign-creation schedule to give you enough time to try out different lines. See Rule 12 for more on testing.

Rule 4: Support the "from" line.

The "from" line tells the recipient who sent the email, and the subject line sells the recipient on opening. If your "from" line lists your company name, you don't have to repeat it in the subject line, which frees up space in the subject line. But do consider branding your subject line with the name of the newsletter, for example, so that it will stand out in the junk folder and your recipients’ overflowing inboxes.

Recent research shows readers often look at the "From" line first when deciding whether to open an email and then the subject line. See the Quick Tip for more information on creating the right "from" line.

Rule 5: List key info first.

Some email clients allow more characters in a subject line than others, but most give you at least 50, including spaces. So, load your key information in that first 50. Also, make sure the cut-off doesn't occur in a crucial word, such as a price or date. Use the EmailLabs subject-line checker see how your subject line renders in the leading desktop and Web email clients.

Rule 6: Open rates don't always measure subject-line success.

Look at the subjects associated with the highest number of conversions, such as registrations, clicks to view newsletter articles, sales or downloads. If you drill down into your Web analytics, you might find some anomalies, such as an email with a relatively low open rate but a high sales-per-order rate. That could mean something in the subject line strongly appealed to a narrow segment of your list and could point the way to a more lucrative segmentation. Remember, your end goal is not necessarily high open rates, but to have subscribers take a specific action. Focus on your end goal.

Rule 7: Personalize.

Personalize subject lines based on users' product or content preferences, interests, past purchases, Web visits or links clicked. Be careful when personalizing on past purchases, however, because the purchase could have been a gift for someone else and might not relate to your reader's real interests. Always make it easy for readers to find and update their data and preferences.

Rule 8: Urgency drives action.

Set a deadline: "Order by midnight tonight;" "Last day to ensure Xmas delivery." Use urgency and deadlines as part of a planned series of emails as well. For example on Monday incorporate “5 Days Left…” and then on Thursday follow it with “Only 24 Hours….”.

Rule 9: Watch those spam filters.

There's a fine line between "catchy" and "spammy." Run your copy through a content checker to identify any spamlike words, phrases or construction. The content checker will tell you which phrases to avoid. Two tricks that could trip a spam filter: subject lines in all capital letters, and using more exclamation points than necessary. (Both look unprofessional, too.) In fact, we recommend against using exclamation points at all if you can avoid it.

Rule 10: "Free" is not evil.

Yes, you can use "free" in a subject line. Just don't make “free” the first word, use it in conjunction with an exclamation point or spell it in all caps (could get your email filtered). People still respond to "free;" so, the increase in orders or other actions will almost always outweigh the messages lost from filtering.

Rule 11. Lead, but don't mislead.

Don't stretch the truth in the subject line or promise more than the email can deliver, or make grand claims that readers will find hard to comply with in order to get a special offer or benefit. Readers will distrust you (and reach for the report-spam button) if your subject line doesn't reflect the email content.

Rule 12. Write and test early and often.

Writing the subject line is often the last and most hurried step in email campaign development. It should be the other way around. As you plan the email campaign, start thinking about what will go into the subject line. That will help you sharpen your campaign's focus and may even change or tweak the offer or article focus.

Ideally, you should test subject lines on a segment of your list, but if you're pressed for time, run them past an informal focus group including your marketing team, others in your department and even folks from outside the department to get a wider view.

Rule 13. Review subject-line performance over your last several campaigns or newsletters.

See which subject lines delivered the action you wanted – the most conversions, the highest average sale per order, the highest click-through rate, etc.

Review your Web analytics reports to see which newsletter article topics draw the most clicks or forwards, which whitepapers get downloaded most often, which brands or departments get the most traffic. This analysis should drive content and product selection strategies, but it can also show you what information is most relevant or useful.

At EmailLabs, we reviewed two years of subject lines and discovered that action-oriented statements that included numbers and "tips" and similar phrases pulled the best. For example, the top 3 most popular EmailLabs/Intevation Report articles from 2005 were "22 Imperatives for Email Marketing Success," "11 Email Marketing Trends for 2005," "15 Tips for Improved Subject Lines."

Rule 14. Continue the conversation.

Sending email more frequently than monthly or quarterly helps you create a conversation with your readers. Your tracking reports should show you what their crucial or hot-button issues are, what kinds of topics gets them opening and clicking more vigorously. Feature those keywords or issues prominently in the subject line where appropriate -- first or second position -- to capture readers' attention.

Additionally, if your email frequency permits it, continue a dialog and content direction you’ve started in previous emails. For example, “Google Agrees to China Censorship” followed by “Google to Testify Before Congress.”

Rule 15. Can you pass the must-open/must-read test?

The days when people opened absolutely everything that landed in their inboxes are long gone. Now, you have to intrigue them. Appeal to their need for information, to be an insider "in the know."

Go back to Rule 14. If you have created a conversation with your readers, a reference to it in your subject will intrigue them into opening your email to see the next installment.

Run a simple test on yourself and others on your team – does the subject line pass these two tests?

1. The must-read test. If a subscriber doesn’t open the email they will feel like they are out of the loop and may have missed an offer that they will regret not taking advantage of.
2. The unbulk bulk-folder test. Simply, if for some reason your email goes into the bulk folder, does the combination of from and subject line wording inspire trust and intrigue to get the recipient to move it into their inboxes?

Conclusion: Much to Learn, Much at Stake

Yes, this seems like a lot of fuss over 50 little characters. But those 50 characters may have the greatest impact on your email's success. It pays to get them right.