1. Be truthful

Don't create a misleading subject line - this will only annoy the reader once they open your email and find that there is something other than what they expected in the content.

2. Keep it short

With only 40-60 characters - that's 6-10 words - to put across your message, any subject line needs to get straight to the point. If you can provide a summary of the contents of the email in your subject line, you are, in effect, pre-selling the reader about the contents of the email.

3. Personalise

Include some form of personal motivation in the subject line – make it relevant – use subject lines based on users' product or content preferences, interests, past purchases, Web visits or links clicked.

4. There is no formula

What works in one campaign might flop with the next. A discount offer should be worded differently from an up-sell, and both are different from a breaking-news announcement. Even if you are sending out similar campaigns, you shouldn't recycle a subject line. You need to stand out each time, yet be familiar to the reader.

5. Support the "from" field

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

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

Remember that your objective is not necessarily high open rates, but to have a high response rate to the call to action.

7. Watch out for SPAM filters

There's a fine line between "catchy" and "spammy." Don't SHOUT (caps lock); it's widely seen as a spammer/scammer trick, can trigger spam filters and isn't considered to be good email etiquette.

8. Write and test early

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.

9. Review subject-line performance

See which subject lines delivered the action you wanted – the most conversions, the highest average sale per order, the highest click-through rate, etc. This analysis should drive content and product selection strategies, but it can also show you what information is most relevant or useful.

10.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 hot topics are. Feature those keywords or issues prominently in the subject line where appropriate to capture readers' attention.