- The best subject lines are pretty straightforward. Your subject line should describe the subject of your email. In email marketing the best subject lines tell what's inside, and the worst sell what's inside.
- Avoid using spammy keywords and phrases. Avoid using ALL CAPS or lots of exclamations points!!!
- If your email is a newsletter, put the name and issue of the newsletter in your subject line. Keep the subject line simple, straightforward and consistent. Always set your subscriber's expectations during the opt-in process about what kinds of emails they'll be receiving. Email newsletters are great for "soft-selling". They build relationships and make constituents feel good about your brand. However, if your recipients signed up for a newsletter, don't expect them to be enthusiastic when you send an email with a subject line like: "Open Now! 10% Discount to the First 500 Customers!"
- Don't confuse newsletters with promotions. If your email is a special promotion, say so in the subject line. Just don't write your subject lines like advertisements. They might seem more creative, but open rates for those emails aren't as high. Unfortunately, most people get so much junk mail in their inbox that anything that even hints of spam is trashed immediately.
- The Subject line of your email is an important part of determining how your email communication will be handled by various email service providers.
Also: Email Subject Lines: 5 Tips to Attract Readers (Nielsen Norman Group)