Here are some things to focus on in writing a good subject line:
1. Be simple and concise.
Simplicity is key when writing e-mail subject lines. Avoid trying to think of a clever subject line and stick to conveying the key information in the e-mail. Keep it short and sweet. 21 percent of people will open an e-mail with a subject line that has six to 10 words, but only 9 percent of people will open an e-mail with 21-25 words .
2. Evoke emotions.
When you make the consumer feel happiness, joy, or excitement, their curiosity is piqued. Eliciting any kind of emotion from the reader tempts the reader into opening your e-mail. Consider including ellipsis, using a list or interesting stat or posing a question .
3. Remember relevant keywords.
Keywords are an important aspect to remember in almost all digital optimization situations. Words that are used a lot in the body of your e-mail should also be present in your subject line. If you've successfully achieved a conversion from your customer, they may want to come back to the e-mail you sent them that prompted the conversion. Utilizing keywords in your subject line will make it easier for the customer to search their inbox and come back to the content they enjoyed.
4. Avoid being sales-y.
Coming off sales-y can rub the customer the wrong way. Try to avoid using any superlatives in your subject line. Words such as “perfect” and “good” reduce open rates by 28 percent . Convey the quality of your work without plugging too much. Additionally, avoid talking about sales. Almost every store has sales, so it's not necessarily noteworthy. Words such as “% off,” “deals,” or “sale ending soon” often trigger the spam folder.
Finally, e-mails that include personalization are highly successful. Using a recipient's first and last name has increased open rates by nearly 33 percent . This is true in all areas of the market except for the legal industry. Engaging the customer through personalization makes the consumer feel a deeper connection with your brand.
E-mail marketing is only one of several potential marketing strategies for your business, but it continues to have a high impact when best practices are followed. Nailing down the subject line is an important first step to success.
 Email Subject Line - Statistics and Trends, Invespcro
 Email Subject Line Success, VistaPrint
 45 words to avoid in your e-mail marketing subject lines, Econsultancy
 Subject Line Data: Choose Your Words Wisely, MailChimp